Screening of largemouth bass-derived Bacillus subtilis and synergistic regulation of growth performance, intestinal health, and resistance against Edwardsiella piscicida infection.

In this study, 49 intestinal bacterial isolates were recovered from healthy Micropterus salmoides, among which five candidate Bacillus subtilis strains were screened via multi-index in vitro evaluation. An 8-week feeding trial covering six treatment groups was conducted to assess growth performance and survival upon Edwardsiella piscicida immersion challenge. Two strains (YB-4 and YB-15) with 95% disease protection rates were selected for further intestinal histological, biochemical and qRT-PCR analyses. Dietary supplementation with YB-4 and YB-15 significantly improved feed conversion efficiency, increased intestinal villus height and muscular layer thickness, enhanced tissue antioxidant capacity, and modulated inflammatory and humoral immune gene expression toward an anti-inflammatory profile. Notably, YB-4 and YB-15 exhibited tissue-specific divergent regulation of key cytokines including TGF-β and IgM, representing a rarely reported strain-dependent immune characteristic of bass-derived B. subtilis. Collectively, YB-4 and YB-15 are promising autochthonous probiotic candidates that simultaneously promote growth, maintain intestinal homeostasis and defend against E. piscicida infection, providing targeted strain-based strategies to reduce antibiotic reliance in largemouth bass intensive aquaculture.