Integrative analyses of probiotics, pathogenic infections and host immune response highlight the importance of gut microbiota in understanding disease recovery in rainbow trout (Oncorhynchus mykiss)

Aims Given the pivotal role played by the gut microbiota in regulating the host immune system, great interest has arisen in the possibility of controlling fish health by modulating the gut microbiota. Hence, there is a need to better understand of the host-microbiota interactions after disease responses to optimize the use of probiotics to strengthen disease resilience and recovery. Methods and Results We tested the effects of a probiotic feed additive in rainbow trout and challenged the fish with the causative agent for enteric red mouth disease, Yersinia ruckeri. We evaluated the survival, host immune gene expression and the gut microbiota composition. Results revealed that provision of probiotics and exposure to Y. ruckeri induced immune gene expression in the host, which were associated with changes in the gut microbiota. Subsequently, infection with Y. ruckeri had very little effect on microbiota composition when probiotics were applied, indicating that probiotics increased stabilisation of the microbiota. Our analysis revealed potential biomarkers for monitoring infection status and fish health. Finally, we used modelling approaches to decipher interactions between gut bacteria and the host immune gene responses, indicating removal of endogenous bacteria elicited by non-specific immune responses. Conclusions We discuss the relevance of these results emphasizing the importance of host-microbiota interactions, including the protective potential of the gut microbiota in disease responses. Significance and Impact of the Study Our results highlight the functional consequences of probiotic-induced changes in the gut microbiota post infection and the resulting host immune response.