Effects of probiotics on growth, the toll-like receptor mediated immune response and susceptibility to Aeromonas salmonicida infection in rainbow trout Oncorhynchus mykiss

Abstract

This study evaluated the effects of probiotics on growth, survival, and toll-like receptors (TLRs)-mediated immune response and susceptibility to Aeromonas salmonicida infection in the rainbow trout (Oncorhynchus mykiss). Probiotic bacterium Bacillus velezensis MVCR2 strain isolated from turbot and Latilactobacillus sakei RDB2 strain isolated from rainbow trout were administered. A 30-day feeding experiment was conducted with three diet treatments: diet supplemented with B. velezensis (BV group), diet supplemented with L. sakei (LS group), and diet without probiotic supplementation as a control. Rainbow trout supplemented with probiotics benefitted by gaining body weight and resistance to A. salmonicida. The expression levels of TLRs (1, 2, 3, 4, 5, 7, 9, and 22) and the related signaling cascade pathways were determined. Upregulation of TLRs (1, 2, 3, 4, 7, 9, and 22) and the signaling cascade genes myd88 and irak4 was detected in different organs after ten days of feeding in the two groups. Interestingly, most of the upregulated TLRs on day ten recovered or even showed downward regulation after 30 days of feeding. Notably, the expression levels of all selected TLRs in the BV and LS groups were significantly upregulated in the kidneys and spleens after the A. salmonicida challenge. Our results suggest that different TLRs are involved in the recognition of exogenous and indigenous beneficial bacteria. Moreover, probiotics were capable of enhancing innate immune function in rainbow trout by training TLR-mediated MyD88-independent pathways, thus helping the host to respond rapidly to pathogen invasion. These findings lay the foundation for future studies on the mechanism by which probiotics regulate fish hosts and provide scientific basis for the use of probiotics in healthy rainbow trout culture.