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For most marine aquaculture species, one of the main bottlenecks is the stable production of high quality juveniles. The high and unpredictable mortality in the first weeks after hatching of marine fish larvae remains a challenging problem that needs to be solved. The severity of the problem differs between species, but cannot be considered adequately solved for any species. Both scientific evidence and experience in hatcheries for a variety of fish, shrimp and shellfish species are accumulating as support for the hypothesis that detrimental fish-microbe interactions are the cause of these problems. Host-microbe interactions in reared fish are still poorly understood, except for a few pathogens, and empirical data of the quality required to test this hypothesis, are lacking. This article provides an overview on the current knowledge of the microbial environment of fish larvae, including methodological aspects to characterize the microbial community (both using culture-dependent and culture-independent methods). Further, the current knowledge of the immunology of fish larvae is reviewed, including recent advances in the understanding of toll-like receptors, inflammatory cytokines, mast cells and piscidins, and the ontogeny of the adaptive immune system. Finally, we provide an overview of the state of the art with respect to steering of microbial communities associated with fish larvae - both steering of community composition and of its activity (e.g. by quorum sensing interference).

The striped catfish (Pangasianodon hypophthalmus) farming system in the Mekong Delta of Viet Nam has been vigorously developed in recent years. It has resulted from expansion of farming areas, application of new farming techniques and an increase of intensification level. As in other developing striped catfish culture venture, health problems including disease outbreaks have been encountered. The use of chemicals and antibiotics in striped catfish farming has increased for prevention and treatment of diseases. These compounds have attracted criticism because of a potential hazard for fish health, environment risk and human consumers. The present study aimed to evaluate the efficiency of different immunostimulating compounds in striped catfish farming in order to reduce the use and impact of antibiotics and chemicals by alternative method of immunostimulation. A combination of in vitro and in vivo approaches was used to investigate the effects of Lipopolysaccharide (LPS) on immune responses of striped catfish. Some potential results of this research were highlighted. The innate immune system of striped catfish was significantly stimulated by LPS injection or feeding through the increase of respiratory burst, lysozyme and complement activities. Total of immunoglobulins was also enhanced in LPS treatment. A challenge test with a common bacterial pathogen (Edwardsiella ictaluri) for striped catfish was performed and documented that LPS could significantly reduce the cumulative mortality of fish, especially in low dose such as 3 mg LPS/kg fish for injection and 0.01% of LPS for oral. Proteomic analysis performed in both approaches suggested that LPS could stimulate the over-expression of several immune proteins such as complement component, lysozyme precursor, transferrin, immunoglobulin,… In addition, the results from in vitro also showed that the response of striped catfish to LPS involved an increase of many protein expression related in cytoskeleton, stress response, (DOCSC03) -- FUNDP, 2013

Pathogenic bacteria communicate with small signal molecules in a process called quorum sensing, and they often use different signal molecules to regulate virulence gene expression. Vibrio campbellii, one of the major pathogens of aquatic organisms, regulates virulence gene expression by a three channel quorum sensing system. Here we show that although they use a common signal transduction cascade, the signal molecules have a different impact on the virulence of the bacterium towards different hosts, i.e. the brine shrimp Artemia franciscana and the commercially important giant freshwater prawn Macrobrachium rosenbergii. These results suggest that the use of multiple types of signal molecules to regulate virulence gene expression is one of the features that allow bacteria to infect different hosts. Our findings emphasize that it is highly important to study the efficacy of quorum sensing inhibitors as novel biocontrol agents under conditions that are as close as possible to the clinical situation.

Knowledge on the importance of associations between bacteria and microalgae in aquatic ecosystems is rather limited at the moment, mostly due to a lack of studies at the molecular and biochemical level of microorganisms. This paper discusses the current knowledge on microalgae–bacteria interactions and their potential impacts on the productivity, efficiency and sustainability of aquaculture. Current findings suggest that the interactions are complex and specific. The release of stimulatory products by bacteria that enhance the growth of microalgae, and vice versa, indicates the existence of mutualistic relations. Other factors, such as signalling between bacteria and microalgae, may also play an important role. Although these interactions may be of significant importance, to date, only a few findings have been reported on the use of consortia consisting of microalgae and bacteria for practical purposes. Interestingly, these results pointed out that a combination of microalgae and bacteria is often better than using either of them alone. Further research is needed to obtain a thorough understanding of the mechanisms behind the interactions between these microorganisms, including the identification of active compounds. This knowledge will enable the selection of appropriate consortia for different applications in aquaculture, including disease control and high and sustainable production of feed.

Background: Vibriosis is one of the most ubiquitous fish diseases caused by bacteria belonging to the genus Vibrio such as Vibrio (Listonella) anguillarum. Despite a lot of research efforts, the virulence factors and mechanism of V. anguillarum are still insufficiently known, in part because of the lack of standardized virulence assays. Methodology/Principal Findings: We investigated and compared the virulence of 15 V. anguillarum strains obtained from different hosts or non-host niches using a standardized gnotobiotic bioassay with European sea bass (Dicentrarchus labrax L.) larvae as model hosts. In addition, to assess potential relationships between virulence and genotypic and phenotypic characteristics, the strains were characterized by random amplified polymorphic DNA (RAPD) and repetitive extragenic palindromic PCR (rep-PCR) analyses, as well as by phenotypic analyses using Biolog's Phenotype MicroArray (TM) technology and some virulence factor assays. Conclusions/Significance: Virulence testing revealed ten virulent and five avirulent strains. While some relation could be established between serotype, genotype and phenotype, no relation was found between virulence and genotypic or phenotypic characteristics, illustrating the complexity of V. anguillarum virulence. Moreover, the standardized gnotobiotic system used in this study has proven its strength as a model to assess and compare the virulence of different V. anguillarum strains in vivo. In this way, the bioassay contributes to the study of mechanisms underlying virulence in V. anguillarum.

Vibrio harveyi is an important aquatic pathogen that produces several virulence factors. In this study, the effect of quorum sensing, bacterial cell-to-cell communication, on the production of the virulence factors caseinase, gelatinase, lipase, hemolysin, and phospholipase, was investigated. The activity of virulence factors was studied through enzymatic plate assays using V. harveyi wild type and mutants with constitutively maximal or minimal quorum sensing activity. The results showed that quorum sensing negatively regulates phospholipase activity as higher activity was observed in mutants with minimal quorum sensing activity than in the mutant with maximal quorum sensing activity. Reverse transcriptase real-time PCR with specific primers revealed that the expression level of three phospholipase genes was 2-fold lower in the mutant with minimal quorum sensing activity than in the mutant with maximal quorum sensing activity. As far as we know, this is the first report of quorum sensing regulation of phospholipase. Finally, caseinase and gelatinase activity were positively regulated by quorum sensing, which is consistent with previous reports, and lipase and hemolysin activity were found to be independent of quorum sensing. Hence, the regulation is different for different virulence factors, with some being either positively or negatively regulated, and others being independent of quorum sensing. This might reflect the need to produce the different virulence factors at different stages during infection.

Quorum sensing, bacterial cell-to-cell communication with small signal molecules such as acyl-homoserine lactones, regulates the virulence of many pathogenic bacteria. Therefore, interfering with quorum sensing is currently being explored as a novel biocontrol strategy to fight bacterial infections. In this study, the effects of 19 micro-algal strains on acyl-homoserine lactone-regulated phenotypes of three reporter strains were investigated. Two freshwater micro-algae inhibited violacein production of quorum sensing reporter strain Chromobacterium violaceum CV026. Further tests using Escherichia coli JB523 showed that micro-algal extracts inhibited or stimulated quorum sensing, depending on the algal strain. One freshwater and five marine algae showed quorum sensing inhibitory activity, whereas two algae stimulated quorum sensing-regulated gene expression. Micro-algal strains that showed inhibitory activity in the previous assays also inhibited acyl-homoserine lactone-regulated bioluminescence in the aquaculture pathogen Vibrio harveyi. The growth of all reporter strains was found to be unaffected by the micro-algal samples. The most promising micro-algal strain was found to be Chlorella saccharophila CCAP211/48, as its extracts inhibited quorum sensing-regulated gene expression in all three reporter strains.

Bacterial disease is one of the most critical problems in commercial aquaculture. Although various methods and treatments have been developed to curb the problem, yet they still have significant drawbacks. A novel and environmental-friendly approach in solving this problem is through the disruption of bacterial communication or quorum sensing (QS). In this communication scheme, bacteria regulate their own gene expression by producing, releasing, and sensing chemical signals from the environment. There seems to be a link between QS and diseases through the regulation of certain phenotypes and the induction of virulence factors responsible for pathogen-host association. Several findings have reported that numerous aquatic organisms such as micro-algae, macro-algae, invertebrates, or even other bacteria have the potential to disrupt QS. The mechanism of action varies from degradation of signals through enzymatic or chemical inactivation to antagonistic as well as agonistic activities. This review focuses on the existing marine organisms that are able to interfere with QS with potential application for aquaculture as bacterial control.

The goal of this study was to test whether it is possible to produce early hatching cysts of Artemia franciscana from San Francisco Bay (SFB) by truncation selection. The starting material was an SFB cyst sample, harvested in nature. After selection of early hatching nauplii had taken place, these selected individuals were cultured to maturity, and the hatching rate of their offspring (F1) was compared with that of the parental generation and with the non-selected control F1. The possible differences in hatching rate were used to estimate heritability. Two different selection experiments were run, accompanied with a number of additional tests studying the influence of the hatching set-up, the hatching temperature and diapause termination using hydrogen peroxide, on the hatching rate. Also the influence of different culture salinities on hatching rate and on possible success of selection was studied. The results revealed an influence of all those abiotic factors on the hatching rate, but also a marked interference of the hatching percentage with the hatching rate. Choosing the individuals based on their own phenotypic values (hatching within a certain time span of hatching incubation) and using those to produce the next generation, revealed a positive response. The selection in different salinities showed a different response. Depending on the strength of the selection pressure, the samples were advanced in time, starting with the highest selection pressure, and ending with the control and the parental sample. In spite of the strong interference of environmental factors, our results suggest that selection of early hatching cysts is possible.