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Among the most important aquaculture resources for our country, salmon and trout stand out. Their production has increased significantly in recent decades, making them two of the most valuable resources in economic terms. However, high aquaculture production has allowed many pathogens to proliferate, causing infectious diseases and significant production losses. Piscirickettsia salmonis is a gram‐negative, facultative intracellular bacterium that is responsible for causing severe disease in a variety of salmonid fish species. Despite the significant impact of P. salmonis on aquaculture, effective treatments for this disease remain limited. Current prevention and control strategies often include antibiotics and vaccines. However, these treatments have shown varying degrees of efficacy. A promising approach involves synthesizing bioactive analog compounds with antibacterial properties. Quinones, secondary metabolites that are abundant in nature, have become a focal point of interest due to their diverse physiological activities, including antibiotic, insecticidal, antifungal, and anticancer properties. In this study, it is shown the synthesis of series 6‐bromo‐7‐arylaminoisoquinoline‐5,8‐quinones, the characterization of these compounds using classical spectroscopic methods such as one‐dimensional nuclear magnetic resonance (NMR), FT‐IR (infrared), mass spectrometry, and the biological activity against Piscirickettsia salmonis. The brominated derivative compounds showed no cytotoxicity at any concentration evaluated. Furthermore, the infectivity of P. salmonis after treatment with the analog compounds indicated that derivatives methyl 6‐bromo‐7‐((4‐methoxyphenyl)amino)‐1,3‐dimethy‐5,8‐dioxo‐5,8‐dihydroisoquinoline‐4‐carboxylate (4b) and methyl 7‐((4′‐amino‐[1,1′‐biphenyl]‐4‐yl)amino)‐6‐bromo‐1,3‐dimethy‐5,8‐dioxo‐5,8‐dihydroisoquinoline‐4‐carboxylate (4g) reduced the bacterial load at 25 μg/mL concentration. [ABSTRACT FROM AUTHOR]

Introduction: Piscirickettsia salmonis, the causative agent of Piscirickettsiosis, poses a significant threat to the Chilean aquaculture industry, resulting in substantial economic losses annually. The pathogen, first identified as specie in 1992, this pathogen was divided into two genogroups: LF-89 and EM-90, associated with different phenotypic mortality and pathogenicity. Traditional genotyping methods, such as multiplex PCR, are effective but limited by their cost, equipment requirements, and the need for specialized expertise. Methods: This study validates Loop-mediated Isothermal Amplification (LAMP) as a rapid and specific alternative for diagnosing P. salmonis infections. We developed the first qPCR and LAMP assay targeting the species-conserved tonB receptor gene (tonB-r, WP_016210144.1) for the specific species-level identification of P. salmonis. Additionally, we designed two genotyping LAMP assays to differentiate between the LF-89 and EM-90 genogroups, utilizing the unique coding sequences Nitronate monooxygenase (WP_144420689.1) for LF-89 and Acid phosphatase (WP_016210154.1) for EM-90. Results: The LAMP assays demonstrated sensitivity and specificity comparable to real-time PCR, with additional benefits including rapid results, lower costs, and simplified operation, making them particularly suitable for field use. Specificity was confirmed by testing against other salmonid pathogens, such as Renibacterium salmoninarum, Vibrio ordalii, Flavobacterium psychrophilum, Tenacibaculum maritimum, and Aeromonas salmonicida, with no cross-reactivity observed. Discussion: The visual detection method and precise differentiation between genogroups underscore LAMP's potential as a robust diagnostic tool for aquaculture. This advancement in the specie detection (qPCR and LAMP) and genotyping of P. salmonis represents a significant step forward in disease management within the aquaculture industry. The implementation of LAMP promises enhanced disease surveillance, early detection, and improved management strategies, ultimately benefiting the salmonid aquaculture sector. [ABSTRACT FROM AUTHOR]

Salmonid rickettsial septicemia (SRS), caused by Piscirickettsia salmonis, has been the most severe health concern for the Chilean salmon industry. The efforts to control P. salmonis infections have focused on using antibiotics and vaccines. However, infected salmonids exhibit limited responses to the treatments. Here, we developed a poly (D, L-lactide-glycolic acid) (PLGA)-nanosystem functionalized with Atlantic salmon IgM (PLGA-IgM) to specifically deliver florfenicol into infected cells. Polymeric nanoparticles (NPs) were prepared via the double emulsion solvent-evaporation method in the presence of florfenicol. Later, the PLGA-NPs were functionalized with Atlantic salmon IgM through carbodiimide chemistry. The nanosystem showed an average size of ~380–410 nm and a negative surface charge. Further, florfenicol encapsulation efficiency was close to 10%. We evaluated the internalization of the nanosystem and its impact on bacterial load in SHK-1 cells by using confocal microscopy and qPCR. The results suggest that stimulation with the nanosystem elicits a decrease in the bacterial load of P. salmonis when it infects Atlantic salmon macrophages. Overall, the IgM-functionalized PLGA-based nanosystem represents an alternative to the administration of antibiotics in salmon farming, complementing the delivery of antibiotics with the stimulation of the immune response of infected macrophages. [ABSTRACT FROM AUTHOR]

Piscirickettsiosis (SRS), caused by Piscirickettsia salmonis, is the main infectious disease that affects farmed Atlantic salmon in Chile. Currently, the official surveillance and control plan for SRS in Chile is based only on the detection of P. salmonis, but neither of its genogroups (LF-89-like and EM-90-like) are included. Surveillance at the genogroup level is essential not only for defining and evaluating the vaccination strategy against SRS, but it is also of utmost importance for early diagnosis, clinical prognosis in the field, treatment, and control of the disease. The objectives of this study were to characterize the spatio-temporal distribution of P. salmonis genogroups using genogroup-specific real-time probe-based polymerase chain reaction (qPCR) to discriminate between LF-89-like and EM-90-like within and between seawater farms, individual fish, and tissues/organs during early infection in Atlantic salmon under field conditions. The spatio-temporal distribution of LF-89-like and EM-90-like was shown to be highly variable within and between seawater farms. P. salmonis infection was also proven to be caused by both genogroups at farm, fish, and tissue levels. Our study demonstrated for the first time a complex co-infection by P. salmonis LF-89-like and EM-90-like in Atlantic salmon. Liver nodules (moderate and severe) were strongly associated with EM-90-like infection, but this phenotype was not detected by infection with LF-89-like or co-infection of both genogroups. The detection rate of P. salmonis LF-89-like increased significantly between 2017 and 2021 and was the most prevalent genogroup in Chilean salmon aquaculture during this period. Lastly, a novel strategy to identify P. salmonis genogroups based on novel genogroup-specific qPCR for LF-89-like and EM-90-like genogroups is suggested.

Piscirickettsiosis is the main cause of mortality in salmonids of commercial importance in Chile, which is caused by Piscirickettsia salmonis, a Gram‐negative, γ‐proteobacteria that can produce biofilm as one of its virulence factors. The Chilean salmon industry uses large amounts of antibiotics to control piscirickettsiosis outbreaks, which has raised concern about its environmental impact and the potential to induce antibiotic resistance. Thus, the use of phytogenic feed additives (PFA) with antibacterial activity emerges as an interesting alternative to antimicrobials. Our study describes the antimicrobial action of an Andrographis paniculate‐extracted PFA on P. salmonis planktonic growth and biofilm formation. We observed complete inhibition of planktonic and biofilm growth with 500 and 400 μg/mL of PFA for P. salmonis LF‐89 and EM‐90‐like strains, respectively. Furthermore, 500 μg/mL of PFA was bactericidal for both evaluated bacterial strains. Sub‐inhibitory doses of PFA increase the transcript levels of stress (groEL), biofilm (pslD), and efflux pump (acrB) genes for both P. salmonis strains in planktonic and sessile conditions. In conclusion, our results demonstrate the antibacterial effect of PFA against P. salmonis in vitro, highlighting the potential of PFA as an alternative to control Piscirickettsiosis. [ABSTRACT FROM AUTHOR]

IntroductionPiscirickettsia salmonis, the causative agent of Piscirickettsiosis, poses a significant threat to the Chilean aquaculture industry, resulting in substantial economic losses annually. The pathogen, first identified as specie in 1992, this pathogen was divided into two genogroups: LF-89 and EM-90, associated with different phenotypic mortality and pathogenicity. Traditional genotyping methods, such as multiplex PCR, are effective but limited by their cost, equipment requirements, and the need for specialized expertise.MethodsThis study validates Loop-mediated Isothermal Amplification (LAMP) as a rapid and specific alternative for diagnosing P. salmonis infections. We developed the first qPCR and LAMP assay targeting the species-conserved tonB receptor gene (tonB-r, WP_016210144.1) for the specific species-level identification of P. salmonis. Additionally, we designed two genotyping LAMP assays to differentiate between the LF-89 and EM-90 genogroups, utilizing the unique coding sequences Nitronate monooxygenase (WP_144420689.1) for LF-89 and Acid phosphatase (WP_016210154.1) for EM-90.ResultsThe LAMP assays demonstrated sensitivity and specificity comparable to real-time PCR, with additional benefits including rapid results, lower costs, and simplified operation, making them particularly suitable for field use. Specificity was confirmed by testing against other salmonid pathogens, such as Renibacterium salmoninarum, Vibrio ordalii, Flavobacterium psychrophilum, Tenacibaculum maritimum, and Aeromonas salmonicida, with no cross-reactivity observed.DiscussionThe visual detection method and precise differentiation between genogroups underscore LAMP's potential as a robust diagnostic tool for aquaculture. This advancement in the specie detection (qPCR and LAMP) and genotyping of P. salmonis represents a significant step forward in disease management within the aquaculture industry. The implementation of LAMP promises enhanced disease surveillance, early detection, and improved management strategies, ultimately benefiting the salmonid aquaculture sector.

For the last 30 years, Piscirickettsia salmonis has caused major economic losses to the aquaculture industry as the aetiological agent for the piscirickettsiosis disease. Replacing the current interventions, based on antibiotics, with natural alternatives (e.g., organic acids) represents a priority. With this study, we aimed to better understand their biological mechanism of action in an in vitro model of infection with salmon epithelial cells (CHSE-214). Our first observation revealed that at the sub-inhibitory concentration of 0.5%, the organic acid blend (Aq) protected epithelial cell integrity and significantly reduced P. salmonis invasion. The MIC was established at 1% Aq and the MBC at 2% against P. salmonis. The sub-inhibitory concentration significantly increased the expression of the antimicrobial peptides Cath2 and Hepcidin1, and stimulated the activity of the innate immune effector iNOS. The increase in iNOS activity also led to higher levels of nitric oxide (NO) being released in the extracellular space. The exposure of P. salmonis to the endogenous NO caused an increase in bacterial lipid peroxidation levels, a damaging effect which can ultimately reduce the pathogen's ability to attach or multiply intracellularly. We also demonstrate that the increased NO release by the host CHSE-214 cells is a consequence of direct exposure to Aq and is not dependent on P. salmonis infection. Additionally, the presence of Aq during P. salmonis infection of CHSE-214 cells significantly mitigated the expression of the pro-inflammatory cytokines IL-1β, IL-8, IL-12, and IFNγ. Taken together, these results indicate that, unlike antibiotics, natural antimicrobials can weaponize the iNOS pathway and secreted nitric oxide to reduce infection and inflammation in a Piscirickettsia salmonis in vitro model of infection. [ABSTRACT FROM AUTHOR]

Piscirickettsia salmonis is the pathogen that most affects the salmon industry in Chile. Large quantities of antibiotics have been used to control it. In search of alternatives, we have developed [Cu(NN1)2]ClO4 where NN1 = 6-((quinolin-2-ylmethylene)amino)-2H-chromen-2-one. The antibacterial capacity of [Cu(NN1)2]ClO4 was determined. Subsequently, the effect of the administration of [Cu(NN1)2]ClO4 on the growth of S. salar, modulation of the immune system and the intestinal microbiota was studied. Finally, the ability to protect against a challenge with P. salmonis was evaluated. The results obtained showed that the compound has an MIC between 15 and 33.9 μg/mL in four isolates. On the other hand, the compound did not affect the growth of the fish; however, an increase in the transcript levels of IFN-γ, IL-12, IL-1β, CD4, lysozyme and perforin was observed in fish treated with 40 μg/g of fish. Furthermore, modulation of the intestinal microbiota was observed, increasing the genera of beneficial bacteria such as Lactobacillus and Bacillus as well as potential pathogens such as Vibrio and Piscirickettsia. Finally, the treatment increased survival in fish challenged with P. salmonis by more than 60%. These results demonstrate that the compound is capable of protecting fish against P. salmonis, probably by modulating the immune system and the composition of the intestinal microbiota. [ABSTRACT FROM AUTHOR]

Salmonid Rickettsial Septicemia (SRS), caused by the bacterium Piscirickettsia salmonis, is the main reason for antibiotic usage in the Chilean aquaculture industry. In 2016, a live attenuated vaccine (ALPHA JECT LiVac® SRS, PHARMAQ AS) was licensed in Chile and has been widely used in farmed salmonids since then. In experimental injection and cohabitation laboratory challenge models, we found that the vaccine is effective in protecting Atlantic salmon (Salmo salar) for at least 15 months against P. salmonis-induced mortality. However, the protection offered by the vaccine is sensitive to temperature during immunization. Fish vaccinated and immunized at 10 °C and above were well protected, but those immunized at 7 °C and 8 °C (the lower end of the temperature range commonly found in Chile) experienced a significant loss of protection. This temperature-dependent loss of effect correlated with the amount of vaccine-strain RNA detected in the liver the first week after vaccination and with in vitro growth curves, which failed to detect any growth at 8 °C. We found that good vaccine efficacy can be restored by exposing fish to 15 °C for the first five days after vaccination before lowering the temperature to 7 °C for the remaining immunization period. This suggests that maintaining the correct temperature during the first few days after vaccination is crucial for achieving a protective immune response with ALPHA JECT LiVac® SRS. Our results emphasize the importance of temperature control when vaccinating poikilothermic animals with live vaccines. [ABSTRACT FROM AUTHOR]

Simple Summary: Farmed Atlantic salmon are routinely exposed to bacterial pathogens, e.g., Piscirickettsia salmonis. Infection by Piscirickettsia sp. leads to a complex array of skin ulcers that can be difficult to treat and increases susceptibility to opportunistic infections. Evidence indicates that bacterial networks residing on salmon skin protect against developing ulcers by excluding pathogen colonization. A collapse of these beneficial interactions is thought to promote susceptibility to pathogen colonization during early stages of infection. We characterized the types and abundances of bacterial constituents on the skin of healthy Atlantic salmon compared with fish suffering from P. salmonis infection to test this hypothesis. The knowledge we gained can be used to optimize methods for early detection and prevention of skin ulcers by disrupting cooperative interactions between pathogenic bacteria. Maintaining the high overall health of farmed animals is a central tenant of their well-being and care. Intense animal crowding in aquaculture promotes animal morbidity especially in the absence of straightforward methods for monitoring their health. Here, we used bacterial 16S ribosomal RNA gene sequencing to measure bacterial population dynamics during P. salmonis infection. We observed a complex bacterial community consisting of a previously undescribed core pathobiome. Notably, we detected Aliivibrio wodanis and Tenacibaculum dicentrarchi on the skin ulcers of salmon infected with P. salmonis, while Vibrio spp. were enriched on infected gills. The prevalence of these co-occurring networks indicated that coinfection with other pathogens may enhance P. salmonis pathogenicity. [ABSTRACT FROM AUTHOR]

Piscirickettsiosis is the most prevalent bacterial disease affecting seawater salmon in Chilean salmon industry. Antibiotic therapy is the first alternative to counteract infections caused by Piscirickettsia salmonis. The presence of bacterial biofilms on materials commonly used in salmon farming may be critical for understanding the bacterial persistence in the environment. In the present study, the CDC Biofilm Reactor® was used to investigate the effect of sub‐ and over‐MIC of florfenicol on both the pre‐formed biofilm and the biofilm formation by P. salmonis under the antibiotic stimuli on Nylon and high‐density polyethylene (HDPE) surfaces. This study demonstrated that FLO, at sub‐ and over‐MIC doses, decreases biofilm‐embedded live bacteria in the P. salmonis isolates evaluated. However, it was shown that in the P. salmonis Ps007 strain the presence of sub‐MIC of FLO reduced its biofilm formation on HDPE surfaces; however, biofilm persists on Nylon surfaces. These results demonstrated that P. salmonis isolates behave differently against FLO and also, depending on the surface materials. Therefore, it remains a challenge to find an effective strategy to control the biofilm formation of P. salmonis, and certainly other marine pathogens that affect the sustainability of the Chilean salmon industry. [ABSTRACT FROM AUTHOR]

Recently, we showed that Atlantic salmon vaccinated against Piscirickettsia salmonis lose their protection upon coinfection with Caligus rogercresseyi (sea lice). However, the causes of the overriding effect of C. rogercresseyi infection have not been elucidated, and the molecular basis of the cellular and humoral immune responses upon C. rogercresseyi infection has not been described for vaccinated salmon. Therefore, we studied changes in the transcription of immune genes in vaccinated Atlantic salmon that were experimentally challenged by co‐infecting them with C. rogercresseyi and P. salmonis. In general, coinfection treatments showed immune gene expression similar to treatments with a single P. salmonis infection, showing a decreased cellular response. However, a high variance was found between individual fish in the case of crucial cellular immune genes, with a few fish reacting overwhelmingly highly compared to the majority. This supports our previous findings on vaccination response variation and reinforces the idea that vaccination failures in the field might be caused by an overwhelming amount of vaccinated fish that display a deficient immune response to the infection. [ABSTRACT FROM AUTHOR]

Piscirickettsia salmonis is the most important health problem facing Chilean Aquaculture. Previous reports suggest that P. salmonis can survive in salmonid macrophages by interfering with the host immune response. However, the relevant aspects of the molecular pathogenesis of P. salmonis have been poorly characterized. In this work, we evaluated the transcriptomic changes in macrophage-like cell line SHK-1 infected with P. salmonis at 24- and 48-hours post-infection (hpi) and generated network models of the macrophage response to the infection using co-expression analysis and regulatory transcription factor-target gene information. Transcriptomic analysis showed that 635 genes were differentially expressed after 24- and/or 48-hpi. The pattern of expression of these genes was analyzed by weighted co-expression network analysis (WGCNA), which classified genes into 4 modules of expression, comprising early responses to the bacterium. Induced genes included genes involved in metabolism and cell differentiation, intracellular transportation, and cytoskeleton reorganization, while repressed genes included genes involved in extracellular matrix organization and RNA metabolism. To understand how these expression changes are orchestrated and to pinpoint relevant transcription factors (TFs) controlling the response, we established a curated database of TF-target gene regulatory interactions in Salmo salar, SalSaDB. Using this resource, together with co-expression module data, we generated infection context-specific networks that were analyzed to determine highly connected TF nodes. We found that the most connected TF of the 24- and 48-hpi response networks is KLF17, an ortholog of the KLF4 TF involved in the polarization of macrophages to an M2-phenotype in mammals. Interestingly, while KLF17 is induced by P. salmonis infection, other TFs, such as NOTCH3 and NFATC1, whose orthologs in mammals are related to M1-like macrophages, are repressed. In sum, our results suggest the induction of early regulatory events associated with an M2-like phenotype of macrophages that drives effectors related to the lysosome, RNA metabolism, cytoskeleton organization, and extracellular matrix remodeling. Moreover, the M1-like response seems delayed in generating an effective response, suggesting a polarization towards M2-like macrophages that allows the survival of P. salmonis. This work also contributes to SalSaDB, a curated database of TF-target gene interactions that is freely available for the Atlantic salmon community.

Piscirickettsiosis, the cause of great losses in Chilean salmonid population, is produced by Piscirickettsia salmonis. The bacterium has developed resistance to treatments over time, including antibiotics and vaccines, due to a variety of evasive strategies, including biofilm formation. The bacterium has genomic islands, clusters of relevant genes that vary between genomes. This work aimed to analyze the relationship between number of pathogenic islands and their virulence genes in complete genomes from different strains of EM and LF genogroups, besides the search for genomic islands with non-virulent factors, like metabolic islands and fitness. Furthermore, we evaluated conservation and variation between strains from different genogroup, considering LF-89 (ATCC) and Psal-001 as reference strains, analyzing genomes through averages by two bioinformatic tools. About 13 pathogenic islands/strain were quantified, with 1 to 47 loci/island with unique behaviors for genogroup. Comparing islands regarding genomes, higher conservation rate was seen in EM genogroup, against oscillating values within LF genogroup strains. Non-virulent factors were also found. Overall, EM genogroup showed greater convergence in island conservation between strains concerning the pathogenic function compared to LF strains. Finally, different transposases were found to support high genomic plasticity and the scarcity of cured genes complicated correct identification.

Salmonid Rickettsial Septicemia (SRS), caused by the bacterium Piscirickettsia salmonis, is the main reason for antibiotic usage in the Chilean aquaculture industry. In 2016, a live attenuated vaccine (ALPHA JECT LiVac® SRS, PHARMAQ AS) was licensed in Chile and has been widely used in farmed salmonids since then. In experimental injection and cohabitation laboratory challenge models, we found that the vaccine is effective in protecting Atlantic salmon (Salmo salar) for at least 15 months against P. salmonis-induced mortality. However, the protection offered by the vaccine is sensitive to temperature during immunization. Fish vaccinated and immunized at 10 °C and above were well protected, but those immunized at 7 °C and 8 °C (the lower end of the temperature range commonly found in Chile) experienced a significant loss of protection. This temperature-dependent loss of effect correlated with the amount of vaccine-strain RNA detected in the liver the first week after vaccination and with in vitro growth curves, which failed to detect any growth at 8 °C. We found that good vaccine efficacy can be restored by exposing fish to 15 °C for the first five days after vaccination before lowering the temperature to 7 °C for the remaining immunization period. This suggests that maintaining the correct temperature during the first few days after vaccination is crucial for achieving a protective immune response with ALPHA JECT LiVac® SRS. Our results emphasize the importance of temperature control when vaccinating poikilothermic animals with live vaccines.

For the last 30 years, Piscirickettsia salmonis has caused major economic losses to the aquaculture industry as the aetiological agent for the piscirickettsiosis disease. Replacing the current interventions, based on antibiotics, with natural alternatives (e.g., organic acids) represents a priority. With this study, we aimed to better understand their biological mechanism of action in an in vitro model of infection with salmon epithelial cells (CHSE-214). Our first observation revealed that at the sub-inhibitory concentration of 0.5%, the organic acid blend (Aq) protected epithelial cell integrity and significantly reduced P. salmonis invasion. The MIC was established at 1% Aq and the MBC at 2% against P. salmonis. The sub-inhibitory concentration significantly increased the expression of the antimicrobial peptides Cath2 and Hepcidin1, and stimulated the activity of the innate immune effector iNOS. The increase in iNOS activity also led to higher levels of nitric oxide (NO) being released in the extracellular space. The exposure of P. salmonis to the endogenous NO caused an increase in bacterial lipid peroxidation levels, a damaging effect which can ultimately reduce the pathogen’s ability to attach or multiply intracellularly. We also demonstrate that the increased NO release by the host CHSE-214 cells is a consequence of direct exposure to Aq and is not dependent on P. salmonis infection. Additionally, the presence of Aq during P. salmonis infection of CHSE-214 cells significantly mitigated the expression of the pro-inflammatory cytokines IL-1β, IL-8, IL-12, and IFNγ. Taken together, these results indicate that, unlike antibiotics, natural antimicrobials can weaponize the iNOS pathway and secreted nitric oxide to reduce infection and inflammation in a Piscirickettsia salmonis in vitro model of infection.

Nutritional immunity regulates the homeostasis of micronutrients such as iron, manganese, and zinc at the systemic and cellular levels, preventing the invading microorganisms from gaining access and thereby limiting their growth. Therefore, the objective of this study was to evaluate the activation of nutritional immunity in specimens of Atlantic salmon (Salmo salar) that are intraperitoneally stimulated with both live and inactivated Piscirickettsia salmonis. The study used liver tissue and blood/plasma samples on days 3, 7, and 14 post-injections (dpi) for the analysis. Genetic material (DNA) of P. salmonis was detected in the liver tissue of fish stimulated with both live and inactivated P. salmonis at 14 dpi. Additionally, the hematocrit percentage decreased at 3 and 7 dpi in fish stimulated with live P. salmonis, unchanged in fish challenged with inactivated P. salmonis. On the other hand, plasma iron content decreased during the experimental course in fish stimulated with both live and inactivated P. salmonis, although this decrease was statistically significant only at 3 dpi. Regarding the immune-nutritional markers such as tfr1, dmt1, and ireg1 were modulated in the two experimental conditions, compared to zip8, ft-h, and hamp, which were down-regulated in fish stimulated with live and inactivated P. salmonis during the course experimental. Finally, the intracellular iron content in the liver increased at 7 and 14 dpi in fish stimulated with live and inactivated P. salmonis, while the zinc content decreased at 14 dpi under both experimental conditions. However, stimulation with live and inactivated P. salmonis did not alter the manganese content in the fish. The results suggest that nutritional immunity does not distinguish between live and inactivated P. salmonis and elicits a similar immune response. Probably, this immune mechanism would be self-activated with the detection of PAMPs, instead of a sequestration and/or competition of micronutrients by the living microorganism. [ABSTRACT FROM AUTHOR]

Simple Summary: In mammals, autophagy plays a fundamental role in the defense against intracellular pathogens; however, in fish, this noncanonical function has not been described. In this context, it was proposed to study whether autophagy was modulated/activated in muscle cells challenged with a bacterial pathogen. Muscle cell cultures were performed and challenged with Piscirickettsia salmonis, the main threat to the salmon industry. Genes associated with immune response and autophagy were evaluated. In addition, the protein content of the LC3-II-specific marker of the autophagic process was evaluated via Western blot. Additionally, genes associated with vesicular traffic and endocytosis were evaluated, finding that P. salmonis promotes these processes. The results show a concomitant modulation of the genes associated with the immune response, vesicular trafficking, and autophagy, suggesting an early intracellular response by the muscle cell against this bacterium. Due to the necessity of seeking and discovering new alternatives and strategies to fight intracellular pathogens in the salmon industry, a better understanding of how autophagy participates in immune system responses may lead to the development of new technologies that allow for the effective control of intracellular pathogens, improving animal welfare and contributing to the sustainability of the global fish industry. Autophagy is a fundamental cellular process implicated in the health of the cell, acting as a cytoplasmatic quality control machinery by self-eating unfunctional organelles and protein aggregates. In mammals, autophagy can participate in the clearance of intracellular pathogens from the cell, and the activity of the toll-like receptors mediates its activation. However, in fish, the modulation of autophagy by these receptors in the muscle is unknown. This study describes and characterizes autophagic modulation during the immune response of fish muscle cells after a challenge with intracellular pathogen Piscirickettsia salmonis. For this, primary cultures of muscle cells were challenged with P. salmonis, and the expressions of immune markers il-1β, tnfα, il-8, hepcidin, tlr3, tlr9, mhc-I and mhc-II were analyzed through RT-qPCR. The expressions of several genes involved in autophagy (becn1, atg9, atg5, atg12, lc3, gabarap and atg4) were also evaluated with RT-qPCR to understand the autophagic modulation during an immune response. In addition, LC3-II protein content was measured via Western blot. The challenge of trout muscle cells with P. salmonis triggered a concomitant immune response to the activation of the autophagic process, suggesting a close relationship between these two processes. [ABSTRACT FROM AUTHOR]

In aquaculture, bacterial pathogens like Francisella noatunensis subsp. noatunensis and Piscirickettsia salmonis significantly impact economic sustainability, causing chronic infections in diverse aquatic environments. Non-coding RNAs (ncRNAs) play crucial roles in bacterial gene regulation, influencing metabolic pathways and stress responses, essential for environmental adaptability. Our comprehensive analysis uncovers a broad spectrum of ncRNAs, both shared across and unique to each pathogen, implicated in essential biological functions such as thiamine biosynthesis, temperature adaptation, and manganese homeostasis. Specifically, the examination of RNA-seq data from the P. salmonis LF-89 strain, representing the LF-89 genogroup, led to the identification of 43 novel trans-small RNAs. This discovery significantly enhances our understanding of the ncRNA-mediated regulatory framework within this genogroup. These novel ncRNAs are suggested to play roles in regulating critical cellular processes, including mobile genetic element management and chromosome partitioning, highlighting their potential influence on bacterial virulence and host-pathogen dynamics. Our findings advance the genomic insight into Francisella noatunensis subsp. noatunensis and Piscirickettsia salmonis , offering a foundation for developing targeted interventions. By elucidating ncRNA roles in pathogenicity, this research contributes to creating innovative strategies to mitigate bacterial diseases in aquaculture, enhancing industry sustainability and economic flexibility. • Analysis ncRNA of F. noatunensis and P. salmonis genomes reveals complex regulatory roles in pathogenicity and adaptation. • Discovery of 43 novel tsRNAs in P. salmonis LF-89 reveals genogroup-specific regulation, enhancing understanding of virulence. • Differential ncRNA in P. salmonis genogroups LF89 and EM90 reveals virulence paths, aiding precise vaccine development. [ABSTRACT FROM AUTHOR]

Abstract The study of the stress responses in bacteria has given us a wealth of information regarding the mechanisms employed by these bacteria in aggressive or even non-optimal living conditions. This information has been applied by several researchers to identify molecular targets related to pathogeny, virulence, and survival, among others, and to design new prophylactic or therapeutic strategies against them. In this study, our knowledge of these mechanisms has been summarized with emphasis on some aquatic pathogenic bacteria of relevance to the health and productive aspects of Chilean salmon farming (Piscirickettsia salmonis, Tenacibaculum spp., Renibacterium salmoninarum, and Yersinia ruckeri). This study will aid further investigations aimed at shedding more light on possible lines of action for these pathogens in the coming years.

Nutritional immunity regulates the homeostasis of micronutrients such as iron, manganese, and zinc at the systemic and cellular levels, preventing the invading microorganisms from gaining access and thereby limiting their growth. Therefore, the objective of this study was to evaluate the activation of nutritional immunity in specimens of Atlantic salmon (Salmo salar) that are intraperitoneally stimulated with both live and inactivated Piscirickettsia salmonis. The study used liver tissue and blood/plasma samples on days 3, 7, and 14 post-injections (dpi) for the analysis. Genetic material (DNA) of P. salmonis was detected in the liver tissue of fish stimulated with both live and inactivated P. salmonis at 14 dpi. Additionally, the hematocrit percentage decreased at 3 and 7 dpi in fish stimulated with live P. salmonis, unchanged in fish challenged with inactivated P. salmonis. On the other hand, plasma iron content decreased during the experimental course in fish stimulated with both live and inactivated P. salmonis, although this decrease was statistically significant only at 3 dpi. Regarding the immune-nutritional markers such as tfr1, dmt1, and ireg1 were modulated in the two experimental conditions, compared to zip8, ft-h, and hamp, which were down-regulated in fish stimulated with live and inactivated P. salmonis during the course experimental. Finally, the intracellular iron content in the liver increased at 7 and 14 dpi in fish stimulated with live and inactivated P. salmonis, while the zinc content decreased at 14 dpi under both experimental conditions. However, stimulation with live and inactivated P. salmonis did not alter the manganese content in the fish. The results suggest that nutritional immunity does not distinguish between live and inactivated P. salmonis and elicits a similar immune response. Probably, this immune mechanism would be self-activated with the detection of PAMPs, instead of a sequestration and/or competition of micronutrients by the living microorganism.

Genome-wide association studies (GWAS) allow the identification of associations between genetic variants and important phenotypes in domestic animals, including disease-resistance traits. Whole Genome Sequencing (WGS) data can help increase the resolution and statistical power of association mapping. Here, we conduced GWAS to asses he facultative intracellular bacterium Piscirickettsia salmonis, which affects farmed rainbow trout, Oncorhynchus mykiss, in Chile using imputed genotypes at the sequence level and searched for candidate genes located in genomic regions associated with the trait. A total of 2130 rainbow trout were intraperitoneally challenged with P. salmonis under controlled conditions and genotyped using a 57K single nucleotide polymorphism (SNP) panel. Genotype imputation was performed in all the genotyped animals using WGS data from 102 individuals. A total of 488,979 imputed WGS variants were available in the 2130 individuals after quality control. GWAS revealed genome-wide significant quantitative trait loci (QTL) in Omy02, Omy03, Omy25, Omy26 and Omy27 for time to death and in Omy26 for binary survival. Twenty-four (24) candidate genes associated with P. salmonis resistance were identified, which were mainly related to phagocytosis, innate immune response, inflammation, oxidative response, lipid metabolism and apoptotic process. Our results provide further knowledge on the genetic variants and genes associated with resistance to intracellular bacterial infection in rainbow trout. [ABSTRACT FROM AUTHOR]

Keywords: aminoisoquinolinquinones; antimicrobial activity; green synthesis; Piscirickettsia salmonis EN aminoisoquinolinquinones antimicrobial activity green synthesis Piscirickettsia salmonis 85 89 5 12/13/22 20230101 NES 230101 The Chilean and international salmon farming industry is mainly threatened by extensive losses caused by infectious and contagious diseases occurring during the production cycle (Figueroa et al., [10]). Results showed that 25 and 50 g/ml of B 2a b , B 2b b , B 2d b , B 2e b , B 2f b , and B 2 h b compounds reduced cell viability >50% in cells. These results indicate that quinone compounds might represent new alternatives to control infectious diseases. In conclusion, it could be speculated that the bactericidal effect of B 2b b compound is due to the observed cytotoxicity of this compound in CHSE-214 cells. [Extracted from the article]

Chile is the second largest producer of salmon (only behind Norway), and high amounts of antimicrobials are used during the marine stage, mainly florfenicol. Intensive antimicrobials use is a relevant concern, prompting the urgent need for use reduction and advancements towards more restricted, efficient application. This review describes the historic and current use of antimicrobials in Chilean salmonid farming. Further covered is the experimental evidence reported to date, thus providing a clearer understanding for the pathogenic nature of Piscirickettsia salmonis—the most important bacterial pathogen and the main target of antimicrobials used in the Chilean industry. This review also assesses research exploring the impacts of antimicrobial treatment on the environment, as well as investigations on the antibiotic susceptibility of salmonid pathogens. This review of the scientific literature contributes to a more unbiased appraisal of the current situation in Chile. Unfortunately, various previous reports dealing with/on the antimicrobial use in the Chilean salmon farming industry have been arrived to insufficiently supported conclusions, thus the present review aims to clarify outdated or poorly substantiated information and assumptions in existing literature. The consequences of antimicrobials in fish farming are covered by this review, which is particularly relevant considering potential impacts on aquatic environments. Focus is given to the selection and spread of drug‐resistant bacteria and associated resistance‐encoding genes. Finally, herein are provided several recommendations from a One Health perspective for decreasing amounts of used antimicrobials, effectively controlling primary bacterial fish pathogens, developing proper prevention strategies and reducing the dissemination of antimicrobial resistance. [ABSTRACT FROM AUTHOR]

The development of fish oral vaccines is of great interest to the aquaculture industry due to the possibility of rapid vaccination of a large number of animals at reduced cost. In a previous study, we evaluated the effect of alginate-encapsulated Piscirickettsia salmonis antigens (AEPSA) incorporated in feed, effectively enhancing the immune response in Atlantic salmon (Salmo salar). In this study, we seek to characterize AEPSA produced by ionic gelation using an aerodynamically assisted jetting (AAJ) system, to optimize microencapsulation efficiency (EE%), to assess microparticle stability against environmental (pH, salinity and temperature) and gastrointestinal conditions, and to evaluate microparticle incorporation in fish feed pellets through micro-CT-scanning. The AAJ system was effective in obtaining small microparticles (d < 20 μm) with a high EE% (97.92%). Environmental conditions (pH, salinity and temperature) generated instability in the microparticles, triggering protein release. 62.42% of the protein content was delivered at the intestinal level after in vitro digestion. Finally, micro-CT-scanning images confirmed microparticle incorporation in fish feed pellets. In conclusion, the AAJ system is effective at encapsulating P. salmonis antigens in alginate with a high EE% and a size small enough to be incorporated in fish feed and produce an oral vaccine. [ABSTRACT FROM AUTHOR]

Piscirickettsia salmonis is the etiological agent of Piscirickettsiosis, a severe disease that affects Atlantic salmon (Salmo salar) farmed in Chile and many other areas (Norway, Scotland, Ireland, Canada and the USA). This study investigated the effects of low‐dose P. salmonis infection (1 × 102 CFU/ml) on Atlantic salmon. In this study, we challenged fish with an isolated representative of the EM‐90 genogroup via intraperitoneal injection for 42 days. Infected fish displayed decreased haematocrit and haemoglobin levels at day 13 post‐infection, indicating erythropenia, haemolysis and haemodilution. Conversely, their white blood cell counts increased on days 13 and 21 post‐infection. Additionally, their iron levels decreased from day 2 post‐infection, indicating iron deficiency and an inability to retrieve stored iron before infection. Their magnesium levels also decreased at day 28 post‐infection, possibly due to osmoregulatory problems. Also, we observed an increase in lactate dehydrogenase activity on days 5, 21, and 28 post‐infection, suggesting early symptoms of hepatotoxicity. Later analyses determined a decrease in plasma glucose levels from day 2 post‐infection. This may be attributed to the hypoxic conditions caused by P. salmonis, leading to an excess utilization of stored carbohydrates. Our results suggest that the blood parameters we studied are useful for monitoring the physiological status of Atlantic salmon infected with P. salmonis. [ABSTRACT FROM AUTHOR]

Background: Endosomally produced by eukaryotic cells, exosomes are microvesicles involved in cell-tocell communication. Exosomes have shown a wide range of therapeutic potential as a drug or vaccine delivery system, and they are useful as biomarkers in several disease processes. Another biological function described is pathogen dissemination through host-derived molecules released during infection, thus modulating the immune response in the host. Results: This work characterizes the exosomal fraction recovered from serum of Piscirickttesia salmonischallenged Salmo salar specimens and from the corresponding non-challenged controls. Exosomes presented a spherical morphology and particle size distribution within 50-125 nm, showing similar parameters in both groups. The mass spectrometry analysis of exosomes isolated at 14 and 21 d post-challenge showed the presence of peptides corresponding to the three proteins of Hsp70/DnaK chaperone system (DnaK, DnaJ, and GrpE). BLAST search of these peptides showed the specificity to P. salmonis. Data are available via ProteomeXchange with identifier PXD023594. Conclusions: The chaperones were found with >95% identity in the core genome when aligned to 73 genomes of P. salmonis. The proteins also showed a high degree of similarity with other microorganisms, where this system has proven to be vital for their survival under stress conditions. The presence of these three proteins in exosomes isolated from challenged fish sera calls for further study into their potential role in bacterium pathogenicity. [ABSTRACT FROM AUTHOR]

Research into Piscirickettsia salmonis biofilms on materials commonly used in salmon farming is crucial for understanding its persistence and virulence. We used the CDC Biofilm Reactor to investigate P. salmonis (LF‐89 and EM‐90) biofilm formation on Nylon, Stainless steel (316L), Polycarbonate and High‐Density Polyethylene (HDPE) surfaces. After 144 h of biofilm visualization by scanning confocal laser microscopy under batch growth conditions, Nylon coupons generated the greatest biofilm formation and coverage compared to Stainless steel (316L), Polycarbonate and HDPE. Additionally, P. salmonis biofilm formation on Nylon was significantly greater (p ≤.01) than Stainless steel (316L), Polycarbonate and HDPE at 288 h. We used Nylon coupons to determine the kinetic parameters of the planktonic and biofilm phases of P. salmonis. The two strains had similar latencies in the planktonic phase; however, LF‐89 maximum growth was 2.5 orders of magnitude higher (Log cell ml−1). Additionally, LF‐89 had a specified growth rate (µmax) of 0.0177 ± 0.006 h−1 and a generation time of 39.2 h. This study contributes to a deeper understanding of the biofilm formation by P. salmonis and elucidates the impact of the biofilm on aquaculture systems. [ABSTRACT FROM AUTHOR]

The intensive salmon farming is associated with massive outbreaks of infections. The use of antibiotics for their prevention and control is related to damage to the environment and human health. Antimicrobial peptides (AMPs) have been proposed as an alternative to the use of antibiotics for their antimicrobial and immunomodulatory activities. However, one of the main challenges for its massive clinical application is the high production cost and the complexity of chemical synthesis. Thus, recombinant DNA technology offers a more sustainable, scalable, and profitable option. In the present study, using an AMPs function prediction methodology, we designed a chimeric peptide consisting of sequences derived from cathelicidin fused with the immunomodulatory peptide derived from flagellin. The designed peptide, CATH-FLA was produced by recombinant expression using an easy pre-purification system. The chimeric peptide was able to induce IL-1β and IL-8 expression in Salmo salar head kidney leukocytes, and prevented Piscirickettsia salmonis -induced cytotoxicity in SHK-1 cells. These results suggest that pre-purification of a recombinant AMP-based chimeric peptide designed in silico allow obtaining a peptide with immunomodulatory activity in vitro. This could solve the main obstacle of AMPs for massive clinical applications. • The first in silico design of chimeric antimicrobial peptide (AMPs) in teleosts. • Chimeric AMP can be recombinantly expressed using an easy pre-purification system. • Chimeric AMP peptide induces a pro-inflammatory response. • Chimeric peptide prevents cytotoxicity induced by infection with P. salmonis. [ABSTRACT FROM AUTHOR]

Abstract Salmonid Rickettsial Septicaemia (SRS), caused by Piscirickettsia salmonis, is the most important infectious disease in the Chilean salmon farming industry. An opportunity to control this disease is to use functional micronutrients to modulate host mechanisms of response to the infection. Since P. salmonis may affect the host antioxidant system in salmonids, particularly that dependent on selenium (Se), we hypothesized that fish’s dietary selenium supplementation could improve the response to the bacterial infection. To address this, we defined a non-antibiotic, non-cytotoxic concentration of selenium to evaluate its effect on the response to in vitro infections of SHK-1 cells with P. salmonis. The results indicated that selenium supplementation reduced the cytopathic effect, intracellular bacterial load, and cellular mortality of SHK-1 by increasing the abundance and activity of host glutathione peroxidase. We then prepared diets supplemented with selenium up to 1, 5, and 10 mg/kg to feed juvenile trout for 8 weeks. At the end of this feeding period, we obtained their blood plasma and evaluated its ability to protect SHK-1 cells from infection with P. salmonis in ex vivo assays. These results recapitulated the observed ability of selenium to protect against infection with P. salmonis by increasing the concentration of selenium and the antioxidant capacity in fish’s plasma. To the best of our knowledge, this is the first report of the protective capacity of selenium against P. salmonis infection in salmonids, becoming a potential effective host-directed dietary therapy for SRS and other infectious diseases in animals at a non-antibiotic concentration.

Abstract Background Vertebrate hosts limit the availability of iron to microbial pathogens in order to nutritionally starve the invaders. The impact of iron deficiency induced by the iron chelator deferoxamine mesylate (DFO) was investigated in Atlantic salmon SHK-1 cells infected with the facultative intracellular bacterium Piscirickettsia salmonis. Results Effects of the DFO treatment and P. salmonis on SHK-1 cells were gaged by assessing cytopathic effects, bacterial load and activity, and gene expression profiles of eight immune biomarkers at 4- and 7-days post infection (dpi) in the control group, groups receiving single treatments (DFO or P. salmonis) and their combination. The chelator appears to be well-tolerated by host cells, while it had a negative impact on the number of bacterial cells and associated cytotoxicity. DFO alone had minor effects on gene expression of SHK-1 cells, including an early activation of IL-1β at 4 dpi. In contrast to few moderate changes induced by single treatments (either infection or chelator), most genes had highest upregulation in the infected groups receiving DFO. The mildest induction of hepcidin-1 (antimicrobial peptide precursor and regulator of iron homeostasis) was observed in cells exposed to DFO alone, followed by P. salmonis infected cells while the addition of DFO to infected cells further increased the mRNA abundance of this gene. Transcripts encoding TNF-α (immune signaling) and iNOS (immune effector) showed sustained increase at both time points in this group while cathelicidin-1 (immune effector) and IL-8 (immune signaling) were upregulated at 7 dpi. The stimulation of protective gene responses seen in infected cultures supplemented with DFO coincided with the reduction of bacterial load and activity (judged by the expression of P. salmonis 16S rRNA), and damage to cultured host cells. Conclusion The absence of immune gene activation under normal iron conditions suggests modulation of host responses by P. salmonis. The negative effect of iron deficiency on bacteria likely allowed host cells to respond in a more protective manner to the infection, further decreasing its progression. Presented findings encourage in vivo exploration of iron chelators as a promising strategy against piscirickettsiosis.

The study of the stress responses in bacteria has given us a wealth of information regarding the mechanisms employed by these bacteria in aggressive or even non-optimal living conditions. This information has been applied by several researchers to identify molecular targets related to pathogeny, virulence, and survival, among others, and to design new prophylactic or therapeutic strategies against them. In this study, our knowledge of these mechanisms has been summarized with emphasis on some aquatic pathogenic bacteria of relevance to the health and productive aspects of Chilean salmon farming (Piscirickettsia salmonis, Tenacibaculum spp., Renibacterium salmoninarum, and Yersinia ruckeri). This study will aid further investigations aimed at shedding more light on possible lines of action for these pathogens in the coming years. [ABSTRACT FROM AUTHOR]

The innate immune system can limit the growth of invading pathogens by depleting micronutrients at a cellular and tissue level. However, it is not known whether nutrient depletion mechanisms discriminate between living pathogens (which require nutrients) and pathogen-associated molecular patterns (PAMPs) (which do not). We stimulated SHK-1 cells with different PAMPs (outer membrane vesicles of Piscirickettsia salmonis "OMVs", protein extract of P. salmonis "TP" and lipopolysaccharides of P. salmonis "LPS") isolated from P. salmonis and evaluated transcriptional changes in nutritional immunity associated genes. Our experimental treatments were: Control (SHK-1 stimulated with bacterial culture medium), OMVs (SHK-1 stimulated with 1μg of outer membrane vesicles), TP (SHK-1 stimulated with 1μg of total protein extract) and LPS (SHK-1 stimulated with 1μg of lipopolysaccharides). Cells were sampled at 15-, 30-, 60- and 120-minutes post-stimulation. We detected increased transcription of zip8 , zip14 , irp1 , irp2 and tfr1 in all three experimental conditions and increased transcription of dmt1 in cells stimulated with OMVs and TP, but not LPS. Additionally, we observed generally increased transcription of ireg-1, il-6 , hamp , irp1 , ft-h and ft-m in all three experimental conditions, but we also detected decreased transcription of these markers in cells stimulated with TP and LPS at specific time points. Our results demonstrate that SHK-1 cells stimulated with P. salmonis PAMPs increase transcription of markers involved in the transport, uptake, storage and regulation of micronutrients such as iron, manganese and zinc. [ABSTRACT FROM AUTHOR]

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host's own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS. [ABSTRACT FROM AUTHOR]

Bacterial cell envelopes play a critical role in host-pathogen interactions. Macromolecular components of these structures have been closely linked to the virulence of pathogens. Piscirickettsia salmonis is a relevant salmonid pathogen with a worldwide distribution. This bacterium is the etiological agent of piscirickettsiosis, a septicemic disease that causes a high economic burden, especially for the Chilean salmon farming industry. Although P. salmonis has been discovered long ago, its pathogenicity and virulence mechanisms are not completely understood. In this work, we present a genetic approach for producing in-frame deletion mutants on genes related to the biosynthesis of membrane-associated polysaccharides. We provide a detailed in vitro phenotype description of knock-out mutants on wzx and wcaJ genes, which encode predicted lipopolysaccharide (LPS) flippase and undecaprenyl-phosphate glucose phosphotransferase enzymes, respectively. We exhibit evidence that the wzx mutant strain carries a defect in the probably most external LPS moiety, while the wcaJ mutant proved to be highly susceptible to the bactericidal action of serum but retained the ability of biofilm production. Beyond that, we demonstrate that the deletion of wzx , but not wcaJ , impairs the virulence of P. salmonis in an intraperitoneally infected Atlantic salmon, Salmo salar , model of piscirickettsiosis. Our findings support a role for LPS in the virulence of P. salmonis during the onset of piscirickettsiosis. [ABSTRACT FROM AUTHOR]

Abstract Salmonid Rickettsial Septicaemia (SRS), caused by Piscirickettsia salmonis, is a severe bacterial disease in the Chilean salmon farming industry. Vaccines and antibiotics are the current strategies to fight SRS; however, the high frequency of new epizootic events confirms the need to develop new strategies to combat this disease. An innovative opportunity is perturbing the host pathways used by the microorganisms to replicate inside host cells through host-directed antimicrobial drugs (HDAD). Iron is a critical nutrient for P. salmonis infection; hence, the use of iron-chelators becomes an excellent alternative to be used as HDAD. The aim of this work was to use the iron chelator Deferiprone (DFP) as HDAD to treat SRS. Here, we describe the protective effect of the iron chelator DFP over P. salmonis infections at non-antibiotic concentrations, in bacterial challenges both in vitro and in vivo. At the cellular level, our results indicate that DFP reduced the intracellular iron content by 33.1% and P. salmonis relative load during bacterial infections by 78%. These findings were recapitulated in fish, where DFP reduced the mortality of rainbow trout challenged with P. salmonis in 34.9% compared to the non-treated group. This is the first report of the protective capacity of an iron chelator against infection in fish, becoming a potential effective host-directed therapy for SRS and other animals against ferrophilic pathogens.

The innate immune system can limit the growth of invading pathogens by depleting micronutrients at a cellular and tissue level. However, it is not known whether nutrient depletion mechanisms discriminate between living pathogens (which require nutrients) and pathogen-associated molecular patterns (PAMPs) (which do not). We stimulated SHK-1 cells with different PAMPs (outer membrane vesicles of Piscirickettsia salmonis “OMVs”, protein extract of P. salmonis “TP” and lipopolysaccharides of P. salmonis “LPS”) isolated from P. salmonis and evaluated transcriptional changes in nutritional immunity associated genes. Our experimental treatments were: Control (SHK-1 stimulated with bacterial culture medium), OMVs (SHK-1 stimulated with 1μg of outer membrane vesicles), TP (SHK-1 stimulated with 1μg of total protein extract) and LPS (SHK-1 stimulated with 1μg of lipopolysaccharides). Cells were sampled at 15-, 30-, 60- and 120-minutes post-stimulation. We detected increased transcription of zip8, zip14, irp1, irp2 and tfr1 in all three experimental conditions and increased transcription of dmt1 in cells stimulated with OMVs and TP, but not LPS. Additionally, we observed generally increased transcription of ireg-1, il-6, hamp, irp1, ft-h and ft-m in all three experimental conditions, but we also detected decreased transcription of these markers in cells stimulated with TP and LPS at specific time points. Our results demonstrate that SHK-1 cells stimulated with P. salmonis PAMPs increase transcription of markers involved in the transport, uptake, storage and regulation of micronutrients such as iron, manganese and zinc.

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host’s own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS.

Bacterial cell envelopes play a critical role in host-pathogen interactions. Macromolecular components of these structures have been closely linked to the virulence of pathogens. Piscirickettsia salmonis is a relevant salmonid pathogen with a worldwide distribution. This bacterium is the etiological agent of piscirickettsiosis, a septicemic disease that causes a high economic burden, especially for the Chilean salmon farming industry. Although P. salmonis has been discovered long ago, its pathogenicity and virulence mechanisms are not completely understood. In this work, we present a genetic approach for producing in-frame deletion mutants on genes related to the biosynthesis of membrane-associated polysaccharides. We provide a detailed in vitro phenotype description of knock-out mutants on wzx and wcaJ genes, which encode predicted lipopolysaccharide (LPS) flippase and undecaprenyl-phosphate glucose phosphotransferase enzymes, respectively. We exhibit evidence that the wzx mutant strain carries a defect in the probably most external LPS moiety, while the wcaJ mutant proved to be highly susceptible to the bactericidal action of serum but retained the ability of biofilm production. Beyond that, we demonstrate that the deletion of wzx, but not wcaJ, impairs the virulence of P. salmonis in an intraperitoneally infected Atlantic salmon, Salmo salar, model of piscirickettsiosis. Our findings support a role for LPS in the virulence of P. salmonis during the onset of piscirickettsiosis.

Piscirickettsiosis (SRS), caused by Piscirickettsia salmonis, is the main infectious disease that affects farmed Atlantic salmon in Chile. Currently, the official surveillance and control plan for SRS in Chile is based only on the detection of P. salmonis, but neither of its genogroups (LF-89-like and EM-90-like) are included. Surveillance at the genogroup level is essential not only for defining and evaluating the vaccination strategy against SRS, but it is also of utmost importance for early diagnosis, clinical prognosis in the field, treatment, and control of the disease. The objectives of this study were to characterize the spatio-temporal distribution of P. salmonis genogroups using genogroup-specific real-time probe-based polymerase chain reaction (qPCR) to discriminate between LF-89-like and EM-90-like within and between seawater farms, individual fish, and tissues/organs during early infection in Atlantic salmon under field conditions. The spatio-temporal distribution of LF-89-like and EM-90-like was shown to be highly variable within and between seawater farms. P. salmonis infection was also proven to be caused by both genogroups at farm, fish, and tissue levels. Our study demonstrated for the first time a complex co-infection by P. salmonis LF-89-like and EM-90-like in Atlantic salmon. Liver nodules (moderate and severe) were strongly associated with EM-90-like infection, but this phenotype was not detected by infection with LF-89-like or co-infection of both genogroups. The detection rate of P. salmonis LF-89-like increased significantly between 2017 and 2021 and was the most prevalent genogroup in Chilean salmon aquaculture during this period. Lastly, a novel strategy to identify P. salmonis genogroups based on novel genogroup-specific qPCR for LF-89-like and EM-90-like genogroups is suggested.

The membrane-anchored and soluble Toll-like Receptor 5 -TLR5M and TLR5S, respectively-from teleost recognize bacterial flagellin and induce the pro-inflammatory cytokines expression in a MyD88-dependent manner such as the TLR5 mammalian orthologous receptor. However, it has not been demonstrated whether the induced signaling pathway by these receptors activate innate effector mechanisms MyD88-dependent in salmonids. Therefore, in this work we study the MyD88 dependence on the induction of TLR5M/TLR5S signaling pathway mediated by flagellin as ligand on the activation of some innate effector mechanisms. The intracellular and extracellular Reactive Oxygen Species (ROS) production and conditioned supernatants production were evaluated in RTS11 cells, while the challenge with Piscirickettsia salmonis was evaluated in SHK-1 cells. Our results demonstrate that flagellin directly stimulates ROS production and indirectly stimulates it through the production of conditioned supernatants, both in a MyD88-dependent manner. Additionally, flagellin stimulation prevents the cytotoxicity induced by infection with P. salmonis in a MyD88-dependent manner. In conclusion we demonstrate that MyD88 is an essential adapter protein in the activation of the TLR5M/TLR5S signaling pathway mediated by flagellin in salmonids, which leads downstream to the induction of innate effector mechanisms, promoting immuno-protection against a bacterial challenge with P. salmonis. • Flagellin induces MyD88-dependent intra/extracellular ROS production in RTS11 cells. • Conditioned supernatants from flagellin stimulated RTS11 cells induces MyD88-dependent ROS. • Flagellin induces MyD88-dependent immune-protection in an in vitro challenge with P. salmonis. [ABSTRACT FROM AUTHOR]

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, the most prevalent disease in salmonid species in Chilean salmonids farms. Many bacteria produce N-acyl-homoserine lactones (AHLs) as a quorum-sensing signal molecule to regulate gene expression in a cell density-dependent manner, and thus modulate physiological characteristics and several bacterial mechanisms. In this study, a fluorescent biosensor system method and gas chromatography-tandem mass spectrometry (GC/MS) were combined to detect AHLs produced by P. salmonis. These analyses revealed an emitted fluorescence signal when the biosensor P. putida EL106 (RPL4cep) was co-cultured with both, P. salmonis LF-89 type strain and an EM-90-like strain Ps007, respectively. Furthermore, the production of an AHL-type molecule was confirmed by GC/MS by both P. salmonis strains, which identified the presence of a N-acetyl-L-homoserine Lactone in the supernatant extract. However, It is suggested that an alternate pathway could synthesizes AHLs, which should be address in future experiments in order to elucidate this important bacterial process. To the best of our knowledge, the present report is the first to describe the type of AHLs produced by P. salmonis.

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, the most prevalent disease in salmonid species in Chilean salmonids farms. Many bacteria produce N -acyl-homoserine lactones (AHLs) as a quorum-sensing signal molecule to regulate gene expression in a cell density-dependent manner, and thus modulate physiological characteristics and several bacterial mechanisms. In this study, a fluorescent biosensor system method and gas chromatography-tandem mass spectrometry (GC/MS) were combined to detect AHLs produced by P. salmonis. These analyses revealed an emitted fluorescence signal when the biosensor P. putida EL106 (RPL4cep) was co-cultured with both, P. salmonis LF-89 type strain and an EM-90-like strain Ps007, respectively. Furthermore, the production of an AHL-type molecule was confirmed by GC/MS by both P. salmonis strains, which identified the presence of a N-acetyl-L-homoserine Lactone in the supernatant extract. However, It is suggested that an alternate pathway could synthesizes AHLs, which should be address in future experiments in order to elucidate this important bacterial process. To the best of our knowledge, the present report is the first to describe the type of AHLs produced by P. salmonis. [ABSTRACT FROM AUTHOR]

During the winter of 2013 and 2016, several Croatian fish farms experienced mortalities in the fry of European sea bass, Dicentrarchus labrax. Affected fish showed abnormal swimming behaviour and reduced appetite, and death ensued several days after the onset of clinical signs of disease. Necropsy revealed pale liver, empty digestive tract, distended gall bladder, and hyperaemia and congestion of the meninges. Routine bacteriological examination tested negative, and virological examination ruled out nodavirus infection. Histological examination revealed multifocal necrosis and extensive inflammation in the brain with abundant cellular debris in the ventricles. Inflammatory cells displayed intra‐cytoplasmic basophilic vacuoles leading to suspicion of Piscirickettsia salmonis infection. Fluorescent in situ hybridization using an oligonucleotide probe targeting Domain Bacterium applied to tissue sections tested positive. The pathogen was identified by 16S rRNA gene sequencing of brain material, and the sequence showed 99% similarity with P. salmonis. This result enabled the design of an oligonucleotide probe specifically targeting P. salmonis. In 2016, P. salmonis was successfully isolated on CHAB from the brain of an affected specimen and identified using 16S rRNA gene sequencing and MALDI‐TOF. This study describes the first outbreak of disease caused by P. salmonis in sea bass in Croatia, while new diagnostic tools will enable further research on its epidemiology and pathogenicity. [ABSTRACT FROM AUTHOR]

Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis, a bacterial disease that affects farmed salmonids, causing high mortalities and significant economic losses in the Chilean salmon farm industry. Given the Chilean native fish species Patagonian blenny, Eleginops maclovinus, lives in the vicinity of salmon farms, it is relevant to clarify the epidemiological role that this species could play in the transmission and/or dissemination of this pathogen. This study aimed to evaluate the bidirectional transmission of P. salmonis between the Patagonian blenny and Oncorhynchus mykiss (rainbow trout), via a cohabitation challenge model. The results of this study demonstrated the transmission of the bacteria from Patagonian blennies to rainbow trout, considering the specific mortality in cohabitant rainbow trout, reaching 46%: the necropsy of these specimens, evidencing the characteristic pathological lesions of the disease and the positive results of the qPCR analysis for P. salmonis, in the same individuals. In contrast, no mortalities of Patagonian blenny specimens were recorded in the challenged experimental groups. This study is the first report showing the horizontal transmission of P. salmonis from a native non‐salmonid species, such as the Patagonian blenny, to a salmonid species, generating the disease and specific mortality in rainbow trout, using a cohabitation challenge. [ABSTRACT FROM AUTHOR]

Salmonid Rickettsial Septicaemia (SRS), caused by Piscirickettsia salmonis, is the most important infectious disease in the Chilean salmon farming industry. An opportunity to control this disease is to use functional micronutrients to modulate host mechanisms of response to the infection. Since P. salmonis may affect the host antioxidant system in salmonids, particularly that dependent on selenium (Se), we hypothesized that fish's dietary selenium supplementation could improve the response to the bacterial infection. To address this, we defined a non-antibiotic, non-cytotoxic concentration of selenium to evaluate its effect on the response to in vitro infections of SHK-1 cells with P. salmonis. The results indicated that selenium supplementation reduced the cytopathic effect, intracellular bacterial load, and cellular mortality of SHK-1 by increasing the abundance and activity of host glutathione peroxidase. We then prepared diets supplemented with selenium up to 1, 5, and 10 mg/kg to feed juvenile trout for 8 weeks. At the end of this feeding period, we obtained their blood plasma and evaluated its ability to protect SHK-1 cells from infection with P. salmonis in ex vivo assays. These results recapitulated the observed ability of selenium to protect against infection with P. salmonis by increasing the concentration of selenium and the antioxidant capacity in fish's plasma. To the best of our knowledge, this is the first report of the protective capacity of selenium against P. salmonis infection in salmonids, becoming a potential effective host-directed dietary therapy for SRS and other infectious diseases in animals at a non-antibiotic concentration. [ABSTRACT FROM AUTHOR]

This study aimed at estimating parameters representing between‐farm transmission of Salmonid Rickettsial Septicaemia (SRS) in Chile, and developing and validating simulation models to predict weekly spread of SRS between farms in Los Lagos (Region 10), using InterSpread Plus. The model parameters were estimated by analyses of the historical SRS outbreak data. The models incorporated time and distance‐dependent transmission kernels, representing the probabilities of waterborne spread of SRS between farms. Seven candidate transmission kernels were estimated, with varying maximum distance of between‐farm SRS spread (15–60 km). Farms were categorized by size (small; medium; large) and species (Coho salmon; Atlantic salmon; rainbow trout). The time that it took a farm to recover from infection was parameterized to be shortest for small Coho farms (median: 7 weeks), followed by medium and large Coho farms (median: 25 weeks), Atlantic salmon farms (median: 42 weeks, any size) and rainbow trout farms (median: 43 weeks, any size). The relative infectiousness parameters of rainbow trout farms were 1.5–6.3 times that of Coho or Atlantic salmon, or those of large farms was 1.3–4.2 times that of small or medium farms. The models predicted SRS prevalence in Region 10 between 2013 and 2015 (79 weeks) with 76.5%–93.0% overall accuracy. The model with a transmission kernel of <20 km (P20) achieved a maximum overall accuracy (93.0%). Within each neighbourhood, the accuracy of P20 varied between 32.4% and 88.1%; 13/20 neighbourhoods had a reasonable temporal agreement between the simulated and actual dynamics of SRS (within 5th–95th percentiles), but 5/20 neighbourhoods underestimated and 2/20 overestimated the SRS spread. The model could be used for evaluation of semi‐global control policies in Region 10, while addition of other factors such as seasonality, ocean currents, and movement of infected fish may improve the model performance at a finer scale. [ABSTRACT FROM AUTHOR]