Histological, immune, and intestine microbiota responses of the intestine of rainbow trout (Oncorhynchus mykiss) to high temperature stress

Fish intestine acts as a multifunctional barrier between the internal and external environments. Rainbow trout (Oncorhynchus mykiss) is a type of cold -water salmonid species which is highly sensitive to temperature changes, but little is known about the systematically impact and regulatory mechanisms of high-temperature exposure on its intestinal health. This study designed three temperature treatment groups of rainbow trout, with 18 degree celsius (elevated, T18) and 21 degree celsius (elevated, T21) as high-temperature stress group, and 15 degree celsius as the control group, to evaluate their effects on intestinal histology, immune response, and microbiota. The results showed that the fish growth was significantly inhibited under high temperature stress, and the susceptibility of fish to A. salmonicida significantly increased, with 100% mortality after 24 h challenge. In addition, histology results showed that both intestinal epithelium cells and goblet cells were significantly affected, leading to pathological damage. In the high temperature treatment groups, the expression of pro-inflammatory cytokines, IL1 beta and IL8, increased while the expression level of IL6 decreased, and HSP90 mRNA related to cellular stress responses increased on day 4. Gut microbiota analysis revealed the microbial diversity decreased at T18 but increased at T21. The Proteobacteria, Firmicutes and Fusobacteriota were dominant phylum in all treatment groups, and the abundance of Proteobacteria decreased while Firmicutes increased in the T18 group. KEGG pathways showed the category "environmental information processing" and "Human diseases" significantly increased at level 1, while "Immune system" and "Replication and repair" significantly decreased at level 2. These findings indicate that heat stress can promote intestinal damage and disrupt the homeostasis of gut microbiota in rainbow trout, providing insight into the crosstalk between gut immunity and microbiota in rainbow trout under heat stress.