ANALYSIS OF PROBIOTIC, INFECTION, AND MICROBIOME IN RAINBOW TROUT. undefined

Background Infectious diseases cause significant production losses in aquaculture every year. Given the pivotal role played by the gut microbiota in regulating the host immune system, interest has arisen in the possibility of monitoring and controlling fish health by modulating the gut microbiota. This illustrates the need for a better understanding of the host-microbiota interactions after disease responses to optimise the use of probiotics for improvement of prophylactic strategies to strengthen disease resilience. Results We used rainbow trout as a model to test the effects of a probiotic feed additive and challenged the fish with the common bacterial pathogen causing enteric redmouth disease (ERM or yersiniosis), Yersinia ruckeri. We evaluated responses by combining data on survival, host immune gene expression and on the gut microbiota composition. Results revealed that: I) Both provision of probiotics and exposure to Y. ruckeri induced immune gene expression in the host associated and changes in the gut microbiota, II) infection Y. ruckeri had very little effect on microbiota composition when probiotics were applied, indicating that probiotics had a microbiota stabilising effect. Furthermore, III) Provision of a Pediococcus probiotic altered host immune responses in the intestine, when exposed to Y. ruckeri, though these alterations had no effect on fish survival, IV) Changes in relative abundance of bacteria revealed potential biomarkers for monitoring infection status and fish health. Finally, IV) we used modelling approaches to decipher interactions between bacterial biomarkers of infection and the host immune gene responses, indicating removal of resident endogenous bacteria elicited by non-specific immune responses. Conclusion We discuss the relevance of these results emphasising the importance of host-microbiota interactions, including the protective potential of the gut microbiota in disease responses. This includes awareness of the functional consequences of probiotic-induced changes in the gut microbiota and the resulting host immune response.