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Atlantic cod gut-derived bacteria - their immunostimulatory functions and phytase activity

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Kjetil Aune


Aquaculture Europe; Trondheim, Norway, 2009-08-14–2009-08-17

Lazado, Carlo C.; Brinchmann, Monica Fengsrud; Kiron, Viswanath; Caipang, Christopher Marlowe

This study aimed to identify potential probiotic bacteria from the gastrointestinal tract of Atlantic cod and determine their immunomodulatory capabilities and the role of one of the bacterial metabolites – phytase. The four bacterial strains were identified with at least 97% homology to the following known bacterial species: Psychrobacter sp. (GP11), Psychrobacter sp. (GP12), Shewanella sp. (GS11) and Pseudomonas sp. (GP21). All bacterial strains showed strong inhibitory activity against V. anguillarum and A. salmonicida. There was a differential expression of immune-response genes related to bacterial defense and inflammation between strains GP11 and GS11. The expression profile of the BPI/LBP gene in HK cells showed peak expression at 12 hpi for GP11 while a significant increase was observed with GS11 at all sampling points. However, significant upregulation of the g-type lysozyme transcript was only observed with GP11, attaining the peak level at 12 hpi. The transcriptional profile of the proinflammatory cytokine, IL- 1_b in HK lls showed the peak level of expression at 12 hpi with GP11 while it occurred at 1 hpi with GS11. Significant expression of IL-8 was only observed with GS11. Phytase activity of the crude enzyme produced by GP21 (97.1 + 2.36 U) was significantly higher than that from GP12 (75.9 + 16.7 U). The incubation of the HK leucocytes with the crude phytase (50 _mg ml-1) resulted in their enhanced proliferation, higher myeloprxidase and acid phosphatase activities. On the other hand, extracellular responses, e.g., respiratory burst activity and hydrogen peroxide production were not enhanced by the exposure to the enzyme. The supernatants obtained from HK leucocytes incubated with the crude bacterial phytase were unable to suppress the growth of A. salmonicida and V. anguillarum. Our study is the first to explore the presence of gastrointestinal tract bacteria that can modulate immune responses in Atlantic cod. The BPI/LBP gene elicited a significant expression during the early stages of incubation with the potential probionts, suggesting that they trigger an acute phase response in the HK. There was an insignificant upregulation of g-type lysozyme in the HK, contrary to our earlier observation in the blood indicating that different mechanisms are involved. The expression of IL-1_b was more pronounced compared to IL-. ur results imply that these bacterial strains can induce the production of pro-inflammatory cytokines in the immune cells of Atlantic cod, but not at levels that could cause cell death. This has to be verified further. We showed that a metabolite of the bacteria, viz. phytase enhanced cell proliferation, myeloperoxidase and acid phosphatase activities. These events point to an increase in the intracellular enzymes for the possible degradation of phagocytosed pathogens. However, antibacterial activity was not suppressed because extracellular responses were not delivered accordingly (eg. H2O2 production and respiratory burst activity). In conclusion, Atlantic cod gastrointestinal tract can be a good source of potential probiotic bacteria having immunomodulatory capabilities.