The effects of MA18/5M on growth performance, gut integrity, and immune response using and Pacific salmonid models.

Microbial management is central to aquaculture's efficiency. MA18/5M has shown promising results promoting growth, modulation of the immune response, and disease resistance in many fishes. However, the mechanisms through which this strain confers health benefits in fish are poorly understood, particularly in Pacific salmonid models. Briefly, the aims of this study were to i) assess the protective effects of MA18/5M by examining gut barrier function and the expression of tight junction (TJ) and immune genes and , and ii) to determine the protective effects of this strain against a common saltwater pathogen, J382. An model of the salmonid gut was employed utilizing the cell line RTgutGC. Barrier formation and integrity assessed by TEER measurements in RTgutGC, showed a significant decrease in resistance in cells exposed only to J382 for 24 h, but pre-treatment with MA18/5M for 48 h mitigated these effects. While MA18/5M did not significantly upregulate tight junction and immune molecules, pre-treatment with this strain protected against pathogen-induced insults to the gut barrier. In particular, the expression of was significantly induced by J382, suggesting that this molecule might play a role in the host response against this pathogen. To corroborate these observations in live fish, the effects of MA18/5M was evaluated in Chinook salmon reared in real aquaculture conditions. Supplementation with MA18/5M had no effect on Chinook salmon growth parameters after 10 weeks. Interestingly, histopathological results did not show alterations associated with MA18/5M supplementation, indicating that this strain is safe to be used in the industry. Finally, the expression pattern of transcripts encoding TJ and immune genes in all the treatments suggest that variation in expression is more likely to be due to developmental processes rather than MA18/5M supplementation. Overall, our results showed that MA18/5M is a safe strain for use in fish production, however, to assess the effects on growth and immune response previously observed in other salmonid species, an assessment in adult fish is needed.