Protective effects and mechanisms of Bacillus subtilis Ya3.1 against Aeromonas hydrophila infection in hybrid sturgeon
Bacterial diseases, particularly Aeromonas hydrophila infections, have become a major constraint in sturgeon aquaculture. This study investigated the protective effects and underlying mechanisms of the host-derived probiotic Bacillus subtilis Ya3.1 in hybrid sturgeon challenged with A. hydrophila. Based on a previous pretreatment trial (fish received Ya3.1 either through feed or water before bacterial challenge), protective responses were assessed by integrating absolute quantitative PCR, serum innate immune assays, intestinal 16S rRNA gene sequencing, and untargeted liver metabolomics. Probiotic pretreatment significantly increased intestinal B. subtilis-associated gene copy numbers, reduced the intestinal A. hydrophila-associated burden after challenge, and markedly improved cumulative survival compared with the infected control. At the immune level, Ya3.1 pretreatment elevated baseline complement components, particularly C3, and partially preserved alkaline phosphatase activity and C3 concentration after infection, indicating enhanced humoral immune resilience.Microbiota analysis showed that A. hydrophila challenge caused pronounced dysbiosis in untreated fish, characterized by altered community diversity and enrichment of opportunistic taxa including Aeromonas, Ralstonia, Halomonas, Acinetobacter, and Chitinophaga. In contrast, probiotic-pretreated fish maintained greater microbial stability and showed suppression of challenge-enriched opportunistic bacteria together with enrichment of Bacillus. Liver metabolomics further revealed that Ya3.1 pretreatment attenuated infection-induced metabolic disturbance and reshaped hepatic metabolic responses, with glycerophospholipid metabolism emerging as a common pathway modulated by both delivery routes.