Effects of Bacillus velezensis AKH-1 in a murine model of Staphylococcus aureus-induced diarrhea are associated with changes in gut microbial composition and intestinal and hepatic markers.

Staphylococcus aureus is an important bacterial pathogen that readily acquires antimicrobial resistance and can cause diarrhea and extraintestinal injury in animals. Probiotic candidates warrant investigation as potential non-antibiotic interventions. This study characterized goat-derived Bacillus velezensis AKH-1 and evaluated its effects in a murine S. aureus challenge model. Species identity was supported by 16S rRNA sequencing and gyrA-rpoB-purH-based multilocus sequence analysis. AKH-1 tolerated acidic and bile-salt conditions, formed biofilms, produced extracellular enzymes, and inhibited multiple indicator pathogens. It was nonhemolytic and DNase negative under the tested conditions. All 14 antimicrobial disks produced measurable inhibition zones, although categorical susceptibility was not assigned because validated species-specific breakpoints are unavailable. Among the 18 resistance determinants screened by PCR, only primers targeting ant(4')-Ia (aadD) yielded an expected-size amplicon. In challenged mice, AKH-1 administration was associated with differences in serum biochemical indices, gut microbial composition, intestinal-barrier markers, and FXR-FGF15-CYP7A1-related measurements. Compared with the infected group, ileal injury scores were significantly lower in the pre-challenge administration group (PING) and numerically lower in the post-challenge administration group (Tx). Liver injury scores were also numerically lower in both groups, although the ileal comparison for Tx and the liver comparisons for both groups did not reach statistical significance. Together, these findings highlight the probiotic-associated traits of AKH-1 and provide an integrated assessment of intestinal, hepatic, and microbial responses, supporting its further evaluation as a livestock-derived probiotic candidate. Building on these exploratory findings, further studies with concurrent challenge controls and matched post-challenge sampling intervals are needed to clarify the contribution of pretreatment to the lower ileal injury score observed in PING. Strain-specific safety assessment, including confirmation and genomic characterization of the putative aadD determinant, alongside bile-acid profiling and functional validation, will further inform the evaluation of AKH-1 as a probiotic candidate and the interpretation of its associated gut-microbiota-liver responses.