Immune modulation by probiotic strains: Quantification of phagocytosis of Aeromonas salmonicida by leukocytes isolated from gut of rainbow trout (Oncorhynchus mykiss) using a radiolabelling assay

Multi-Omics Analysis Reveals the Regulatory Mechanism of Different Probiotics on Growth Performance and Intestinal Health of Salmo trutta (S. trutta)

Probiotics play an important role in animal production, providing health benefits to the host by improving intestinal microbial balance. In this study, we added three different probiotics, Saccharomyces cerevisiae (SC), Bacillus licheniformis (BL), and lactic acid bacteria (LAB), and compared them with the control group (CON), to investigate the effects of probiotic supplementation on growth performance, gut microbiology, and gut flora of S. trutta. Our results showed that feeding probiotics improved the survival, growth, development, and fattening of S. trutta. Additionally, probiotic treatment causes changes in the gut probiotic community, and the gut flora microorganisms that cause significant changes vary among the probiotic treatments. However, in all three groups, the abundance of Pseudomonas, Acinetobacter, and Rhizophagus bacterial genera was similar to that in the top three comparative controls. Furthermore, differences in the composition of intestinal microbiota among feed types were directly associated with significant changes in the metabolomic landscape, including lipids and lipid-like molecules, organic acids and derivatives, and organoheterocyclic compounds. The probiotic treatment altered the gut microbiome, gut metabolome, and growth performance of S. trutta. Using a multi-omics approach, we discovered that the addition of probiotics altered the composition of gut microbiota, potentially leading to modifications in gut function and host phenotype. Overall, our results highlight the importance of probiotics as a key factor in animal health and productivity, enabling us to better evaluate the functional potential of probiotics.

Rosmarinic acid alone or in combination with Lactobacillus rhamnosus ameliorated ammonia stress in the rainbow trout, Oncorhynchus mykiss: growth, immunity, antioxidant defense and liver functions

Rosmarinic acid (RS) and Lactobacillus rhamnosus (LR) were added singularly or in combination to rainbow trout (Oncorhynchus mykiss) diets to test their efficacy in the protection against ammonia stress. Fish (31.4±0.6 g) were randomly allocated to six groups in three replicates, as follows: T1: basic food as control, T2: LR with a concentration of 1.5 × 108 CFU/g, T3: LR with a concentration of 3 × 108 CFU/g, T4: 1 g RS/kg, T5: 3 g RS/kg, and T6: 1.5 × 108 CFU/g LR + 1 g RS/kg and T7: 3 × 108 CFU/g LR + 3 g RS/kg. After 60 days feeding, fish exposed to 0ammonia stress. After the feeding period, the supplemented fish had the highest final body weight (FW), weight gain (WG), and specific growth rate (SGR), and the lowest feed conversion ratio (FCR) as compared with the control group (P<0.05). Amylase, protease and lipase activities were noticed markedly higher in fish supplemented with 1.5 × 108 CFU/g LR + 1 g RS/kg and 1.5 × 108 CFU/g LR diets compared to the control (P<0.05). Generally, fish in supplemented diets, particularly T2 and T6 groups, had the highest lysozyme, alternative complement activity (ACH50), total Ig, nitroblue tetrazolium test (NBT), myeloperoxidase (MPO), complement component 3 (C3), complement component 4 (C4), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx). On the other hand, T2 and T6 groups had the lowest malondialdehyde (MDA), glucose, and cortisol concentrations as well as alanine aminotransferase (ALT), alkaline phosphatase (ALP), lactate dehydrogenase (LDH) enzyme levels when were compared with the control (P<0.05). After ammonia stress, fish in the supplemented groups, particularly T2 and T6, generally showed significantly higher values of lysozyme, ACH50, total Ig, NBT, MPO, C3, C4, SOD, CAT, GPx and lower levels of MDA, glucose, cortisol, ALT, ALP, LDH when compared with the control (P<0.05). In conclusion, a combined administration of RS and L. rhamnosus effectively improved growth performance and health status as well as enhanced the resistance of rainbow trout against ammonia toxicity.

Effects of two host-associated probiotics Bacillus mojavensis B191 and Bacillus subtilis MRS11 on growth performance, intestinal morphology, expression of immune-related genes and disease resistance of Nile tilapia (Oreochromis niloticus) against Streptococcusiniae

The intensification and diversification of production systems have increased the incidence of diseases, which are usually treated with antibiotics. However, its use should be restricted due to the increasing prevalence of antibiotic-resistant bacteria. Probiotics represent therefore an alternative environmentally friendly strategy for improving growth and disease resistance in aquaculture. Considering that host-derived probiotics may offer greater advantages than those from other environments in terms of safety and efficacy, two potential host-associated probiotic strains (Bacillus mojavensis B191 and Bacillus subtilis MRS11) were used in the present study, which were previously isolated from intestinal mucus of Nile tilapia (Oreochromis niloticus). This study was conducted to assess the effects of dietary administration of two Bacillus strains on growth performance, intestinal morphology, immunity, and disease resistance of Nile tilapia. A total of 375 fish were randomly divided into five groups in triplicate. Nile tilapia were fed a basal diet (control group) or a basal diet supplemented with Bacillus mojavensis B191 (BM) or Bacillus subtilis MRS11 (BS) spores at different concentrations of 1 × 10⁶ (BM6 and BS6, respectively) and 1 × 10⁸ (BM8 and BS8, respectively) CFU/g of feed for 60 days. Moreover, the survival rate of tilapia upon challenge with Streptococcus iniae was determined following the feeding trial. After the feeding trial, the growth performances were significantly improved in all probiotic-fed groups, with the BS8 group being the highest. Light and electron microscopy observations revealed elevated goblet cells, intestinal villus length (except BM8), microvilli length, microvilli density, and perimeter ratio increase in the intestine of all probiotic-fed groups compared with the control group. Regarding the expression analysis, HSP70 gene was only up-regulated in the BM8 group and a general trend of up-regulation of some immune-related cytokines (TGF-β, IL-10, TNF-α and IL-1β) was observed in all probiotic-fed groups. Likewise, the best protection against Streptococcus iniae was observed in the BS8 group, followed by BS6, BM6 and BM8 groups. Altogether, dietary probiotic supplementation with BS8 and BM6 may improve growth performance, intestinal morphology, immunity, and disease resistance in Nile tilapia.

Immunomodulatory action of Lactococcuslactis

Fermented foods are gaining popularity due to health-promoting properties with high levels of nutrients, phytochemicals, bioactive compounds, and probiotic microorganisms. Due to its unique fermentation process, Lactococcus lactis plays a key role in the food business, notably in the manufacturing of dairy products. The superior biological activities of L. lactis in these functional foods include anti-inflammatory and immunomodulatory capabilities. L. lactis boosted growth performance, controlled amino acid profiles, intestinal immunology, and microbiota. Besides that, the administration of L. lactis increased the rate of infection clearance. Innate and acquired immune responses would be upregulated in both local and systemic compartments, resulting in these consequences. L. lactis is often employed in the food sector and is currently being exploited as a delivery vehicle for biological research. These bacteria are being eyed as potential candidates for biotechnological applications. With this in mind, we reviewed the immunomodulatory effects of different L. lactis strains.

Probiotics in tilapia (Oreochromis niloticus) culture: Potential probiotic Lactococcus lactis culture conditions

Tilapia is one of the most extensively farmed fish on a global scale. Lately, many studies have been carried out to select and produce probiotics for cultured fish. Bacteria from the genera Bacillus, Lactiplantibacillus (synonym: Lactobacillus), and Lactococcus are the most widely studied with respect to their probiotic potential. Among these microorganisms, Lactococcus lactis has outstanding prospects as a probiotic because it is generally recognized as safe (GRAS) and has previously been shown to exert its probiotic potential in aquaculture through different mechanisms, such as competitively excluding pathogenic bacteria, increasing food nutritional value, and enhancing the host immune response against pathogenic microorganisms. However, it is not sufficient to simply select a microorganism with significant probiotic potential for commercial probiotic development. There are additional challenges related to strategies involving the mass production of bacterial cultures, including the selection of production variables that positively influence microorganism metabolism. Over the last ten years, L. lactis production in batch and fed-batch processes has been studied to evaluate the effects of culture temperature and pH on bacterial growth. However, to gain a deeper understanding of the production processes, the effect of hydrodynamic stress on cells in bioreactor production and its influence on the probiotic potential post-manufacturing also need to be determined. This review explores the trends in tilapia culture, the probiotic mechanisms employed by L. lactis in aquaculture, and the essential parameters for the optimal scale-up of this probiotic.

Hot water extract of Pleurotus pulmonarius stalk waste enhances innate immune response and immune-related gene expression in red hybrid tilapia Oreochromis sp. following challenge with pathogen-associated molecular patterns

In aquaculture, commercial fish such as red hybrid tilapia are usually raised at high density to boost the production within a short period of time. This overcrowded environment, however, may cause stress to the cultured fish and increase susceptibility to infectious diseases. Antibiotics and chemotherapeutics are used by fish farmers to overcome these challenges, but this may increase the production cost. Studies have reported on the potential of mushroom polysaccharides that can act as immunostimulants to enhance the immune response and disease resistance in fish. In the current study, hot water extract (HWE) from mushroom stalk waste (MSW) was used to formulate fish feed and hence administered to red hybrid tilapia to observe the activation of immune system. Upon 30 days of feeding, the fish were challenged with pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharides (LPS) and polyinosinic:polycytidylic acid (poly (I:C)) to mimic bacterial and viral infection, respectively. HWE supplementation promoted better feed utilisation in red hybrid tilapia although it did not increase the body weight gain and specific growth rate compared to the control diet. The innate immunological parameters such as phagocytic activity and respiratory burst activity were significantly higher in HWE-supplemented group than that of the control group following PAMPs challenges. HWE-supplemented diet also resulted in higher mRNA transcription of il1b and tnfa in midgut, spleen and head kidney at 1-day post PAMPs injection. Tlr3 exhibited the highest upregulation in the HWE fed fish injected with poly (I:C). At 3-days post PAMPs injection, both ighm and tcrb expression were upregulated significantly in the spleen and head kidney. Results showed that HWE supplementation enhances the immune responses of red hybrid tilapia and induced a higher serum bactericidal activity against S. agalactiae.

Mechanisms Used by Probiotics to Confer Pathogen Resistance to Teleost Fish

Probiotics have been defined as live microorganisms that when administered in adequate amounts confer health benefits to the host. The use of probiotics in aquaculture is an attractive bio-friendly method to decrease the impact of infectious diseases, but is still not an extended practice. Although many studies have investigated the systemic and mucosal immunological effects of probiotics, not all of them have established whether they were actually capable of increasing resistance to different types of pathogens, being this the outmost desired goal. In this sense, in the current paper, we have summarized those experiments in which probiotics were shown to provide increased resistance against bacterial, viral or parasitic pathogens. Additionally, we have reviewed what is known for fish probiotics regarding the mechanisms through which they exert positive effects on pathogen resistance, including direct actions on the pathogen, as well as positive effects on the host.

Effective isolation of GALT cells: Insights into the intestine immune response of rainbow trout (Oncorhynchus mykiss) to different bacterin vaccine preparations

The teleost gut is a multifunction complex structure that plays a pivotal immunological role in homeostasis and the maintenance of health, in addition to digestion of food and/or nutrient absorption. In vitro examination of the intestine leucocyte repertoire has the potential to aid our understanding of gut immune competence and allows a rapid screen of host-microorganism interactions in different immunological contexts. To explore this possibility, in the present study we investigated the response of isolated gut leucocytes to 4 bacterins of Aeromonas salmonicida, prepared from different strains, combinations and strains grown in different environments, in comparison to a Yersinia ruckeri bacterin for which a commercial/effective oral booster vaccine has been developed. To aid this study we also optimized further our method of GALT cell isolation from rainbow trout, so as to avoid mechanical clearance of the intestine contents. This drastically increased the cell yield from ~12 × 10⁶ to ~210 × 10⁶/fish with no change in the percent cell viability over time or presence of transcripts typical of the key leucocyte types needed for the study of immune modulation (i.e. T- and B-cells, dendritic cells and macrophages). A wide array of immune transcripts were modulated by the bacterins, demonstrating the diversity of GALT cell responses to bacterial stimulation. Indeed, the GALT leucocyte responses were sensitive enough to distinguish the different bacterial species, strains and membrane proteins, as seen by distinct kinetics of immune gene expression. However, the response of the GALT cells was often relatively slow and of a low magnitude compared to those of PBL. These results enhance our knowledge of the gut biocapacity and help validate the use of this model for screening of oral vaccine candidates.

Characterization of the gut microbiota in early life stages of pikeperch Sander lucioperca

This study characterized the gastrointestinal microbiome of nine juvenile farmed pikeperch Sander lucioperca using a metagenomics approach based on bacterial 16S rRNA gene sequencing. Potential changes in the gut microbiota during 2 months of S. lucioperca juvenile life were investigated. Results revealed that gut microbiota was dominated by Proteobacteria (95-92%), while other phyla Firmicutes (1-1·5%) and Actinobacteria (0·9-1·5%) were less abundant. At the family level, fish-gut microbiota were dominated by Enterobacteriaceae, which constituted c. 83% of all DNA sequence reads. Such a situation was present in all of the examined fish except one, which showed a different proportion of particular microbial taxa than the other fish. In this fish, a higher relative abundance (%) of Fusobacteria (21·0%), Bacteroidetes (9·5%) and Firmicutes (7·5%) was observed. There were no significant differences in the gut microbiome structure at different stages of development in the examined fish. This may indicate that Proteobacteria inhabiting the gut microbiota at an early stage of life are a necessary component of the pikeperch microbiome that may support proper nutrition of the fish. The information obtained on the gut microbiome could be useful in determining juvenile S. lucioperca health and improving rearing conditions by welfare monitoring in aquaculture.

Synbiotic dietary supplement affects growth, immune responses and intestinal microbiota of Apostichopus japonicus

A feeding experiment was conducted to investigate the effects of dietary administration of synbiotic with Bacillus lincheniformis WS-2 (CGMCC No. 12813) and alginate oligosaccharides (AOS) on the growth, innate immune response, and intestinal microbiota of the sea cucumber Apostichopus japonicus and its resistance to Vibrio infection. Sea cucumbers were given a control diet (non-supplemented), pro diet (basal diet plus 1 × 10⁹ cfu (g diet)^-1B. lincheniformis WS-2), syn diet (basal diet plus 1 × 10⁹ cfu (g diet)^-1B. lincheniformis WS-2 and 10 g (kg diet) ^-1 AOS) or pre diet (basal diet plus 10 g (kg diet) ^-1 AOS) over a period of 60 days, and the growth performance and various innate immune parameters of the animals were evaluated after 30 and 60 days of feeding. No significant difference in growth performance was observed between the group fed with the syn and the group fed with the pro diet, but both these groups exhibited significant (P < 0.05) enhancement in growth performance compared to the control group. At the same time, both syn and pro diets also resulted in the animals having significantly higher levels of amylase, protease and alginate lyase activities compared to the con diet. Individuals fed with the syn or pro diet showed enhanced levels of various immune enzyme activities, compared to those fed with the con diet. At the end of the growth period, the sea cucumbers were challenged with Vibrio splendidus via intraperitoneal injection. The survival rates of sea cucumbers fed with the syn, pro or pre diet were significantly improved compared to that of sea cucumbers fed with the con diet, with sea cucumbers fed with synbiotic having the highest survival. In addition, increased proportions of Bacillus and Lactococcus were found in the intestinal tract of sea cucumbers fed with the syn diet (9.5% and 7.3%) compared to those of sea cucumbers fed with the pro diet (6.1% and 4.6%), con diet (4.0% and 3.4%), or pre diet (5.2% and 6.8%) after 60 days of feeding. Furthermore, the proportion of Vibrio in the intestinal tracts of sea cucumbers fed with the pro diet (2%) or syn diet (3.1%) was lower than that of sea cucumbers fed with the con diet (5.5%) or pre diet (3.8%), although no significant difference was detected between the pro diet and syn diet groups (P > 0.05). Overall, the results suggested that dietary synbiotic consisting of Bacillus lincheniformis and alginate oligosaccharides (AOS) could have positive benefit for sea cucumber aquaculture.

Effects of intracellular products of Bacillus subtilis VSG1 and Lactobacillus plantarum VSG3 on cytokine responses in the head kidney macrophages of Labeo rohita

The efficiency of intracellular products (ICPs) of the probiotics Bacillus subtilis VSG1 and Lactobacillus plantarum VSG3 in stimulating cytokine responses in the head kidney (HK) macrophages of Labeo rohita was investigated. The HK macrophages were incubated with ICPs and lipopolysaccharide (LPS), and the responses of cytokine genes, namely interleukin-10 (IL-10), IL-1β, IL-12p35, IL-12p40, IL-18, tumour necrosis factor-α (TNF-α), nuclear factor kappa B (NF-κB), cyclo-oxygenase-2 (COX-2), interferon-1 (IFN-1), and IFN-γ were assessed by quantitative real-time PCR (qRT-PCR) at 2, 6, 12, 24, and 48 h post-stimulation (hps). Among the proinflammatory cytokines, a strong increase in the gene expression of IL-1β and TNF-α was displayed mainly at 2-6 hps with ICPs, as compared to that of the positive control (LPS) or the negative control (PBS) (P < 0.05). However, COX-2 and NF-κB showed higher expression at 2 and 24 hps, and 6-24 hps with ICPs, respectively. Antiviral cytokines IFN-1 and IFN-γ displayed strong expressions (P < 0.05) at 6-12 hps, and 12-24 hps with ICPs, respectively. Upregulation of the anti-inflammatory cytokine, IL-10, was recorded at 6-24 hps with ICPs, as compared to that controls. Expressions of cell-mediated immune factor genes (IL-12p35, IL-12p40, and IL-18) were also significantly upregulated at different time points, except 48 hps, in HK macrophages stimulated with ICPs. Furthermore, enhanced cellular (phagocytic activity and nitroblue tetrazolium assay) and humoral (lysozyme) immune parameters in stimulated cells confirmed the induction of the inflammatory response. Therefore, the results of this in vitro study indicate that the ICPs of B. subtilis VSG1 or L. plantarum VSG3 have great potential for stimulating the cytokine responses in fish, and are thereby potential immunostimulants to fish. Further studies could be conducted to explore its suitability as an adjuvant vaccine in aquaculture.

Evolution of probiotics in aquatic world: Potential effects, the current status in Egypt and recent prospectives

The increase in the human population in addition to the massive demand for protein of animal origin forced the authorities to seek for additional sources of feed supplies. Aquaculture is the world worth coming expansion to compensate the shortage in animal protein. Feed in aquaculture plays an important role in the production cycle and exert threshold on both practical and economic aspects. Feed additive sectors are expanding day after day to achieve better growth and health for fish and shrimp and to meet the potential requirements of the culturists. Probiotic proved its successes in human and animal feeding practices and recently gained attention in aquaculture; it has beneficial effects in diseases control and competes with various environmental stressors as well as to promote the growth of the cultured organisms. Probiotics have the privilege to manipulate the non-specific innate immunity among fishes, hence help them into resist many pathogenic agents and are actively used worldwide. The present review is an informative compilation of the probiotics, their mode of action and their useful effects on fishes. The review also highlights the status of probiotics in aquaculture of Egypt, probiotic recent prospective for the possible role of probiotics in fish external and internal environment.

Probiotics, immunostimulants, plant products and oral vaccines, and their role as feed supplements in the control of bacterial fish diseases

There is a rapidly increasing literature pointing to the success of probiotics, immunostimulants, plant products and oral vaccines in immunomodulation, namely stimulation of the innate, cellular and/or humoral immune response, and the control of bacterial fish diseases. Probiotics are regarded as live micro-organisms administered orally and leading to health benefits. However, in contrast with the use in terrestrial animals, a diverse range of micro-organisms have been evaluated in aquaculture with the mode of action often reflecting immunomodulation. Moreover, the need for living cells has been questioned. Also, key subcellular components, including lipopolysaccharides, have been attributed to the beneficial effect in fish. Here, there is a link with immunostimulants, which may also be administered orally. Furthermore, numerous plant products have been reported to have health benefits, namely protection against disease for which stimulation of some immune parameters has been reported. Oral vaccines confer protection against some diseases, although the mode of action is usually linked to humoral rather than the innate and cellular immune responses. This review explores the relationship between probiotics, immunostimulants, plant products and oral vaccines.

Probiotics and prebiotics associated with aquaculture: A review

There is a rapidly growing literature, indicating success of probiotics and prebiotics in immunomodulation, namely the stimulation of innate, cellular and humoral immune response. Probiotics are considered to be living microorganisms administered orally and lead to health benefits. These Probiotics are microorganisms in sufficient amount to alter the microflora (by implantation or colonization) in specific host's compartment exerting beneficial health effects at this host. Nevertheless, Prebiotics are indigestible fiber which enhances beneficial commensally gut bacteria resulting in improved health of the host. The beneficial effects of prebiotics are due to by-products derived from the fermentation of intestinal commensal bacteria. Among the many health benefits attributed to probiotics and prebiotics, the modulation of the immune system is one of the most anticipated benefits and their ability to stimulate systemic and local immunity, deserves attention. They directly enhance the innate immune response, including the activation of phagocytosis, activation of neutrophils, activation of the alternative complement system, an increase in lysozyme activity, and so on. Prebiotics acting as immunosaccharides directly impact on the innate immune system of fish and shellfish. Therefore, both probiotics and prebiotics influence the immunomodulatory activity boosting up the health benefits in aquatic animals.

Cytokine expression in head-kidney leucocytes of European sea bass (Dicentrarchus labrax L.) after incubation with the probiotic Vagococcus fluvialis L-21

The European sea bass (Dicentrarchus labrax L.) is one of the most extensively farmed marine fish in the Mediterranean sea. Under the high-density condition, common in aquaculture, the infectious diseases can cause significant economic losses. Probiotics are presented as an alternative to antibiotics for the control of aquaculture diseases. This study used real-time PCR to investigate in vitro the dynamic of expression of immune-related genes in sea bass after incubation with live and inactivated (heat and Uv-light) probiotic Vagoccus fluvialis L-21 at different times (T1, T12, T24, T48). The immune associated genes, interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin 10 (IL-10), TumourTumour necrosis factor- (TNF-), ciclo-oxigenase-2 (COX-2), caspase-3 (Casp-3) and Mx were studied in head-kidney (HK) leucocytes of sea bass after incubation with the probiotic strain. Transcript of proinflammatory cytokines (IL-1, TNF-, COX-2) was highly up-regulated after 1 h of incubation with the probiotic strain V. fluvialis L-21. We found statistically significant difference in pick of expression of TNF-, after 1 h of incubation with Uv-light inactivated probiotic strain. The COX-2 expression was highly up-regulated at all times studied, with the exception of 12 and 24 h post incubation for the Uv-light inactivated bacteria. Transcript of IL-10 and Casp-3 showed the higher statistically significant differences of expression after 48 h post incubation with live bacteria. In the contrast, sea bass HK leucocytes expressed Mx at 12 and 48 h without statistically differences among treatments. Our results suggest that V. fluvialis L-21 is able to stimulate in vitro some immune-related genes associated with the early inflammatory response. Future studies in vivo are necessary to clarify this process in sea bass.

Comparison between intestinal and non-mucosal immune functions of rainbow trout, Oncorhynchus mykiss

Since mucosal surfaces represent major portals of entry for pathogens, its associated immune system is important to protect the organism. In this paper, we compared at the cellular and molecular levels intestinal leukocyte suspensions with their head kidney (HK) or peripheral blood (PBL) counterparts to highlight characteristics of intestinal immune functions in healthy rainbow trout. These studies show that intestinal phagocytes are less activated by yeast cells but when they are activated they can ingest as many yeast cells as their HK counterparts. A natural cytotoxic activity could be detected which is twice higher in intestinal than in HK leukocyte preparations. This natural cytotoxic activity is correlated with the expression of transcripts encoding the natural killer enhancement factor (NKEF). Intestinal leukocytes did not respond to an in vitro mitogenic stimulation performed under classical culture conditions. And finally, a high expression of CD8α transcripts was observed in gut leukocyte preparations, suggesting that the intestine could contain a high proportion of T cells expressing the αα homodimeric form of CD8. This kind of comparison on nonimmunized fish provides better knowledge on basal immune functions in the intestine to, analyze later on, immune responses induced by an antigenic stimulation.

Teleost intestinal immunology

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.

Probiotics and immunity: a fish perspective

Probiotics are usually live microorganisms which when administered in adequate amounts confer a health benefits on host. Nowadays, probiotics are also becoming an integral part of the aquaculture practices to obtain high production. The common probiotics that are used for aquaculture practices include Lactobacillus, Lactococcus, Leuconostoc, Enterococcus, Carnobacterium, Shewanella, Bacillus, Aeromonas, Vibrio, Enterobacter, Pseudomonas, Clostridium, and Saccharomyces species. The involvement of probiotics in nutrition, disease resistance and other beneficial activities in fish has proven beyond any doubt. Among the numerous health benefits attributed to probiotics, modulation of immune system is one of the most commonly purported benefits of the probiotics and their potency to stimulate the systemic and local immunity under in vitro and in vivo conditions is noteworthy. Different probiotics either monospecies or multispecies supplementation can eventually elevate phagocytic, lysozyme, complement, respiratory burst activity as well as expression of various cytokines in fish. Similarly, probiotics can stimulate the gut immune system of fish with marked increase in the number of Ig(+) cells and acidophilic granulocytes. Furthermore, mono-bacterial association studies (with non-probiotic bacterial strains) in gnotobiotic fish also indicate the up-regulation of various immune related genes. Though the exact mode of action of probiotics is yet to be established in any animal including fish, probiotics often exert host specific and strain specific differences in their activities. Various factors like source, type, dose and duration of supplementation of probiotics can significantly affect the immunomodulatory activity of probiotics. The review is therefore, aiming to highlight the immunomodulatory activity of probiotics and also to evaluate the factors that regulate for the optimum induction of immune responses in fish.

A transferable 20-kilobase multiple drug resistance-conferring R plasmid (pKL0018) from a fish pathogen (Lactococcus garvieae) is highly homologous to a conjugative multiple drug resistance-conferring enterococcal plasmid

Lactococcus garvieae, the causative agent of lactococcosis, has evolved strains that are highly resistant to antibiotics. Here, the 20,034-bp sequence of L. garvieae conjugative plasmid pKL0018 was determined. It contained two ermB genes and one tetS gene and a backbone more than 96% identical to that of pRE25, an Enterococcus faecalis plasmid from dry sausage.

Protection of rainbow trout (Oncorhynchus mykiss) from lactococcosis by probiotic bacteria

We analysed the effect of probiotic supplementation on the control of lactococcosis in rainbow trout. Probiotic strains Leuconostoc mesenteroides CLFP 196 and Lactobacillus plantarum CLFP 238 were administered orally to fish for 30 days at 10(7) CFU g(-1) feed. Thirty days after the start of the probiotic feeding, fish were challenged with Lactococcus garvieae. Probiotic supplementation reduced fish mortality significantly, from 78% in the control group to 46-54% in the probiotic groups.

Monospecies and multispecies probiotic formulations produce different systemic and local immunostimulatory effects in the gilthead seabream (Sparus aurata L.)

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.

A review on the interactions between gut microbiota and innate immunity of fish

Although fish immunology has progressed in the last few years, the contribution of the normal endogenous microbiota to the overall health status has been so far underestimated. In this context, the establishment of a normal or protective microbiota constitutes a key component to maintain good health, through competitive exclusion mechanisms, and has implications for the development and maturation of the immune system. The normal microbiota influences the innate immune system, which is of vital importance for the disease resistance of fish and is divided into physical barriers, humoral and cellular components. Innate humoral parameters include antimicrobial peptides, lysozyme, complement components, transferrin, pentraxins, lectins, antiproteases and natural antibodies, whereas nonspecific cytotoxic cells and phagocytes (monocytes/macrophages and neutrophils) constitute innate cellular immune effectors. Cytokines are an integral component of the adaptive and innate immune response, particularly IL-1 beta, interferon, tumor necrosis factor-alpha, transforming growth factor-beta and several chemokines regulate innate immunity. This review covers the innate immune mechanisms of protection against pathogens, in relation with the installation and composition of the normal endogenous microbiota in fish and its role on health. Knowledge of such interaction may offer novel and useful means designing adequate therapeutic strategies for disease prevention and treatment.

Enhancement of the immune response and protection induced by probiotic lactic acid bacteria against furunculosis in rainbow trout (Oncorhynchus mykiss)

We analysed the effect of probiotic strains on the cellular and humoral immune responses of rainbow trout (Oncorhynchus mykiss), and their capacity to prevent furunculosis during a challenge trial. Probiotic strains (Lactococcus lactis ssp. lactis CLFP 100, Leuconostoc mesenteroides CLFP 196, and Lactobacillus sakei CLFP 202) were administered orally to fish for 2 weeks at 10(6) CFU g(-1) of feed. In comparison to untreated control fish, the phagocytic activity of head kidney leukocytes and the alternative complement activity in serum were significantly greater in all probiotic groups at the end of the second week. With the exception of the group fed with Lactobacillus sakei, superoxide anion production was also significantly increased in the probiotic groups. Analysis of lysozyme activity did not exhibit any significant difference in the probiotic and control groups. Fifteen days after the start of the probiotic feeding, fish were challenged with Aeromonas salmonicida ssp. salmonicida. The fish supplemented with probiotics exhibited survival rates ranging from 97.8% to 100%, whereas survival was 65.6% in fish not treated with the probiotics. These results demonstrate that probiotic supplementation to fish can reduce the severity of furunculosis, and suggest that this reduction may be associated with enhanced humoral and cellular immune response.

Changes in intestinal microbiota and humoral immune response following probiotic administration in brown trout (Salmo trutta)

We studied the effect of several lactic acid bacteria (LAB) on the humoral response of brown trout (Salmo trutta). LAB groups (Lactococcus(Lc.)lactisssp.lactis,Lactobacillus(Lb.)sakeiandLeuconostoc(Leu.)mesenteroides) were administered orally at 106colony-forming units/g feed to brown trout for 2 weeks, after which fish were switched to an unsupplemented feed. Blood and intestinal samples were taken from the onset of feeding supplemented diets at 1, 2, 3 and 4 weeks. During the LAB-feeding period,Lc. lactisssp.lactis,Lb. sakeiandLeu. mesenteroidespersisted in the fish intestines, but the number of LAB slowly decreased in the intestines after changing to the unsupplemented diet. OnlyLb. lactisssp.lactisandLeu. mesenteroideswere detected at levels above 1 × 102colony-forming units/g at the end of the fourth week. In comparison to untreated control fish, the alternative complement activity in the serum was found to be significantly greater in all LAB groups at the end of the second week. Groups supplemented withLc. lactisssp.lactisandLeu. mesenteroidesexhibited an elevated level of lysozyme activity at the end of the third week, but the group supplemented withLb. sakeidid not exhibit any significant change in lysozyme activity. Serum immunoglobulin levels were higher compared with the control group, but there was no significant difference between the LAB and control groups.