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The aquatic environment, independent of their host, is more favorable to pathogenic bacteria than the terrestrial environment. Consequently, pathogenic bacteria can reach very high densities around aquatic animals and can cause high mortality. The conventional approach, such as antibiotics, has minimal effectiveness. Additionally, due to the emergence of (multiple) resistance, their use is under intense scientific and public scrutiny. Hence, there is a need for the development of alternative control techniques, with an emphasis on prevention, which is likely to be more cost-effective. In this study, a potential bacterial strain Cytobacillus firmus was isolated from polluted river sediment and characterized using a comprehensive range of techniques including biochemical, 16S rRNA sequencing and antibiogram assay. The pathogenicity of the bacteria was tested in vivo on Labeo rohita fingerlings found as non-pathogenic. Further, the bacteria were found to synthesize silver nanoparticles (AgNPs) using AgNO3 as a substrate. The obtained AgNPs were characterized by various methods, including UV-vis spectroscopy, FTIR (Fourier-transform infrared spectroscopy), and Transmission Emission Microscopy (TEM). The study found that the AgNPs were 20 nm in size on average. The antimicrobial activity of synthesized AgNPs was examined against the model freshwater pathogenic bacteria, Edwardsiella tarda and both the MIC (Minimum Inhibitory Concentration) and MBC (Minimum Bactericidal Concentration) were 0.156 µM, while biofilm inhibition activity was also observed at 0.156 µM. The AgNPs showed no haemolytic activity at 0.313 µM. Our findings suggest that C. firmus mediated bacteriogenic AgNPs modulate the activity of common pathogenic bacteria E. tarda. The thoroughness of our research process gives us confidence in the potential of applying AgNPs in aquaculture as a considerable strategy to control the E. tarda infection. [ABSTRACT FROM AUTHOR]

Intestinal parasitic infections caused by helminths are globally distributed and are a major cause of morbidity worldwide. Parasites may modulate the virulence, gut microbiota diversity and host responses during infection. Despite numerous works, little is known about the complex interaction between parasites and the gut microbiota. In the present study, the complex interplay between parasites and the gut microbiota was investigated. A total of 12 bacterial strains across four major families, including Enterobacteriaceae, Morganellaceae, Flavobacteriaceae, and Pseudomonadaceae, were isolated from Channa punctata, infected with the nematode species Aporcella sp., Axonchium sp., Tylencholaimus mirabilis, and Dioctophyme renale. The findings revealed that nematode infection shaped the fish gut bacterial microbiota and significantly affected their virulence levels. Nematode-infected fish bacterial isolates are more likely to be pathogenic, with elevated hemolytic activity and biofilm formation, causing high fish mortality. In contrast, isolates recovered further from non-parasitised C. punctata were observed to be non-pathogenic and had negligible hemolytic activity and biofilm formation. Antibiogram analysis of the bacterial isolates revealed a disproportionately high percentage of bacteria that were either marginally or multidrug resistant, suggesting that parasitic infection-induced stress modulates the gut microenvironment and enables colonization by antibiotic-resistant strains. This isolation-based study provides an avenue to unravel the influence of parasitic infection on gut bacterial characteristics, which is valuable for understanding the infection mechanism and designing further studies aimed at optimizing treatment strategies. In addition, the cultured isolates can supplement future gutmicrobiome studies by providing wet lab specimens to compare (meta)genomic information discovered within the gut microenvironment of fish. [ABSTRACT FROM AUTHOR]

Aquatic food production system raises aquatic organisms including fish, shellfish and seaweeds for human consumption and associated value chains. Moreover, as the global human population continues to expand at a high rate and is expected to reach over 9 billion by 2030, developing a cost-efficient production method is a significant challenge in the future development of the aquatic food production industry to provide food and nutritional security with high-quality animal protein. Recent advancements in biotechnological tools and approaches provided a new toolset that can be used to design and optimize the existing processes such as food preservation, fermentation, packaging, quality control and setting proper guidelines to manufacture and process genetically modified fish. At the same time, with the refinement of technology, these are becoming easier applicable and transferable to several other aquatic species production systems. These trends have resulted in exploiting new and unconventional microbial systems with sophisticated properties, which render promising results in the production industry. Here, we highlight the recent advances in the newly emerging biotechnological technology in the production of fish and fish products and discuss the potential of these tools as a sustainable platform for centuries to come with a significant impact on the aquatic food production industry. [ABSTRACT FROM AUTHOR]

A case of severe mortality in farmed Labeo rohita was investigated to characterize the causative agent. We identified the bacterial strain as Aeromonas veronii isolated from the gut of infected L. rohita by biochemical assay, scanning electron microscopy and 16S rRNA gene sequence analysis. The in vivo challenge experiment showed that the LD50 of A. veronii was 2.2 × 104 CFU/fish. Virulence gene investigation revealed that the isolated A. veronii possesses Aerolysin, Cytotoxic enterotoxin, Serine protease, Dnase and Type III secretion system genes. The isolated strain was resistant to two antibiotics (ampicillin and dicloxacillin) while susceptible to 22 other antibiotics. The study further revealed that A. veronii induced both stresses along with non-specific and specific immune responses marked by elevated cortisol HSP70, HSP90 and IgM levels in the treated L. rohita fingerlings. Although the bacterial pathogen enhances the immune response, the negative effect on fish, including stress, and high mortality, create concern and a need for A. veronii management in L. rohita farms. The knowledge gained from this study would facilitate future research aimed at assessing the pathogenicity of A. veronii, with an emphasis on microbial disease management in other farmed fish species. [ABSTRACT FROM AUTHOR]

In this study, we evaluated gamete quality parameters of mature male koi carp ( Cyprinus carpio ) exposed to three different concentrations (1, 10, and 100 µg/L) of di-(2-ethylhexyl) phthalate (DEHP). After 60 days of exposure, there was a significant decrease in the gonadosomatic index (GSI) of males exposed to 10 and 100 µg/L of DEHP. Histological analysis of the testes revealed impaired histoarchitecture, including inflammatory cells, intratubular vacuoles, and swollen seminiferous tubules in treatment groups. Gamete quality parameters like sperm production, motility, spermatocrit, and sperm density values were significantly decreased at the 10 and 100 µg/L concentrations. Biochemical compositions, including glucose, cholesterol, and total protein levels, were significantly changed in the treatment groups. Similarly, the ionic compositions of seminal fluid (Na, K, Ca, and Mg) also varied in the treatment groups. Furthermore, the 11-ketotestosterone levels were decreased, and the 17-β estradiol levels were increased in the DEHP-treated groups. The mRNA expression levels of reproduction-related genes, including Fshr, Lhr, Ar, Erα, and Erβ, were significantly changed in the DEHP-treated males in a dose-dependent manner. In conclusion, the findings of this study confirmed that environmentally relevant exposure to DEHP may contribute to a decline in the gamete quality of male fishes.