Teleosts clearly have a more diffuse gut associated lymphoid system, which is morphological and functional clearly different from the mammalian GALT. All immune cells necessary for a local immune response are abundantly present in the gut mucosa of the species studied and local immune responses can be monitored after intestinal immunization. Fish do not produce IgA, but a special mucosal IgM isotype seems to be secreted and may (partly) be the recently described IgZ/IgT. Fish produce a pIgR in their mucosal tissues but it is smaller (2 ILD) than the 4-5 ILD pIgR of higher vertebrates. Whether teleost pIgR is transcytosed and cleaved off in the same way needs further investigation, especially because a secretory component (SC) is only reported in one species. Teleosts also have high numbers of IEL, most of them are CD3-ɛ+/CD8-α+ and have cytotoxic and/or regulatory function. Possibly many of these cells are TCRγδ cells and they may be involved in the oral tolerance induction observed in fish. Innate immune cells can be observed in the teleost gut from first feeding onwards, but B cells appear much later in mucosal compartments compared to systemic sites. Conspicuous is the very early presence of putative T cells or their precursors in the fish gut, which together with the rag-1 expression of intestinal lymphoid cells may be an indication for an extra-thymic development of certain T cells. Teleosts can develop enteritis in their antigen transporting second gut segment and epithelial cells, IEL and eosinophils/basophils seem to play a crucial role in this intestinal inflammation model. Teleost intestine can be exploited for oral vaccination strategies and probiotic immune stimulation. A variety of encapsulation methods, to protect vaccines against degradation in the foregut, are reported with promising results but in most cases they appear not to be cost effective yet. Microbiota in fish are clearly different from terrestrial animals. In the past decade a fast increasing number of papers is dedicated to the oral administration of a variety of probiotics that can have a strong health beneficial effect, but much more attention has to be paid to the immune mechanisms behind these effects. The recent development of gnotobiotic fish models may be very helpful to study the immune effects of microbiota and probiotics in teleosts.

The interactions between the endogenous gut microbiota and the fish host are integral in mediating the development, maintenance and effective functionality of the intestinal mucosa and gut associated lymphoid tissues (GALTs). These microbial populations also provide a level of protection against pathogenic visitors to the gastrointestinal (GI) tract and aid host digestive function via the production of exogenous digestive enzymes and vitamins. Manipulation of these endogenous populations may provide an alternative method to antibiotics to control disease and promote health management. Applications of probiotics for Mediterranean teleosts can stimulate immune responses, enhance growth performance, feed utilisation, digestive enzyme activities, antioxidant enzyme activities, gene expression, disease resistance, larval survival, gut morphology, modulate GI microbiota and mediate stress responses. Although considerably less information is available regarding prebiotic applications for Mediterranean teleosts, prebiotics also offer benefits with regards to improving immune status and fish production. Despite the promising potential benefits demonstrated in current literature, obtaining consistent and reliable results is often difficult due to our incomplete understanding of indigenous fish GI microbiota and their subsequent host interactions which mediate and drive both localised and systemic host immunological responses. Additionally, the probiotic and prebiotic (biotics) mechanisms which mediate host benefits at the mucosal interface are poorly understood. Future studies focused on these interactions utilising gnotobiotic techniques should provide a better understanding of how to extract the full potential of biotic applications to promote immune function of Mediterranean teleosts.

The microbiome plays an important role in lipid metabolism but how the introduction of probiotic communities affects host lipid metabolism is poorly understood. Using a multidisciplinary approach we addressed this knowledge gap using the zebrafish model by coupling high-throughput sequencing with biochemical, molecular and morphological analysis to evaluate the changes in the intestine. Analysis of bacterial 16S libraries revealed that Lactobacillus rhamnosus was able to modulate the gut microbiome of zebrafish larvae, elevating the abundance of Firmicutes sequences and reducing the abundance of Actinobacteria. The gut microbiome changes modulated host lipid processing by inducing transcriptional down-regulation of genes involved in cholesterol and triglycerides metabolism (fit2, agpat4, dgat2, mgll, hnf4α, scap, and cck) concomitantly decreasing total body cholesterol and triglyceride content and increasing fatty acid levels. L. rhamnosus treatment also increased microvilli and enterocyte lengths and decreased lipid droplet size in the intestinal epithelium. These changes resulted in elevated zebrafish larval growth. This integrated system investigation demonstrates probiotic modulation of the gut microbiome, highlights a novel gene network involved in lipid metabolism, provides an insight into how the microbiome regulates molecules involved in lipid metabolism, and reveals a new potential role for L. rhamnosus in the treatment of lipid disorders.

Aim:  To assess Pediococcus acidilactici as a dietary supplement for on‐growing red tilapia (Oreochromis niloticus). Methods and Results:  Tilapia were fed either a control diet or control diet supplemented with Ped. acidilactici at 107 CFU g−1 for 32 days. Ped. acidilactici colonized the intestinal tract and significantly affected the intestinal microbial communities. PCR‐DGGE revealed direct antagonism of gastric Ped. acidilactici with an endogenous uncultured bacterium during a period of reverting to nonsupplemented feeding. Light microscopy revealed that gut integrity and leucocyte levels were unaffected by Ped. acidilactici; however, blood leucocyte levels and serum lysozyme activity were elevated after 14‐days' feeding. No significant improvements in growth performance were observed at the end of the trial (day 32), but survival was significantly higher in the probiotic group. Conclusions:  The study demonstrates that oral supplementation of Ped. acidilactici modulates intestinal bacterial communities in on‐growing red tilapia and also stimulates some aspects of the nonspecific immune response. Significance and Impact of the study:  To our knowledge this is the first study assessing the effects of probiotics on the gut microbiota of tilapia using culture‐independent methods. Such methods are crucial to understand the mechanisms which underpin and mediate host benefits.

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 effects of the oral administration of heat-inactivated Lactobacillus delbrüeckii ssp. lactis and Bacillus subtilis, individually or combined, on gilthead seabream immune responses were investigated both systemically and locally in the gut. In a first experiment, seabream (65 g) were fed for 3 weeks different diets supplemented with 1 x 10(7)CFU g(-1)Lactobacillus, 1 x 10(7)CFU g(-1)Bacillus, or 0.5 x 10(7)CFU g(-1)Lactobacillus plus 0.5 x 10(7)CFU g(-1)Bacillus. Controls were fed non-supplemented diet. Six fish per group were sampled at the end of the trial and some humoral and cellular systemic innate immune parameters were evaluated. Feeding the mixture of the two killed bacteria species significantly increased natural complement, serum peroxidase and phagocytic activities compared with controls. In a second experiment, juvenile seabream (13 g) were fed for 3 weeks the same experimental diets and total serum IgM and numbers of gut IgM(+) cells and acidophilic granulocytes were evaluated. All these parameters were significantly higher in the multispecies probiotic group compared to monospecies and control fed groups. The advantages provided by administration of killed probiotic bacteria as well as multispecies versus monospecies formulations are discussed in light of the results obtained and for their possible application in aquacultural practices.

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