Genomic and functional characterization of Bacillus siamensis AFSC23 isolated from the gut of European conger (Conger conger) as a probiotic candidate for aquaculture

The intestinal microbiota of fish represents a valuable source of antimicrobial and health-promoting bacteria. In this context, we report for the first time the isolation of Bacillus siamensis strain AFSC23 from the intestine of Conger conger. The strain was comprehensively characterized using an integrated genomic and functional approach to assess its safety and probiotic potential in aquaculture. The strain exhibited resistance to lysozyme (100 mg/L), gastrointestinal conditions, bile salts (0.3%), high salinity (11%), promising cell surface properties, coupled with inhibitory activity against A. salmonicida, V. anguillarum, P. damselae subsp. damselae and P. damselae subsp . piscicida. Genome analysis revealed the presence of several probiotic gene markers and biosynthetic gene clusters encoding bacteriocins and other antimicrobial compounds. Safety assessment demonstrated susceptibility to most tested antibiotics, with no detectable hemolytic, lipolytic, or DNase activity and it was not able to degrade mucin. Analysis of virulence factors and mobile genetic determinants did not reveal detectable safety concerns associated with the strain. Moreover, extracellular products were non-toxic, with no cytotoxicity observed in Epithelioma papulosum cyprini cells, and in vivo assays indicated no substantial adverse effects in Ruditapes philippinarum under the evaluated conditions. These findings highlight the potential of B. siamensis AFSC23 as a promising probiotic candidate for sustainable aquaculture.