<i>Bifidobacterium bifidum</i> G9‐1 Survives in the Intestinal Environment and Influences the Gut Microbiota Despite the Presence of Antimicrobials

Probiotics can help prevent antibiotic-associated diarrhea; however, those lacking antimicrobial resistance may be ineffective during antimicrobial treatment. Here, we aimed to examine the effects of antimicrobials on the probiotic strain Bifidobacterium bifidum G9-1 (BBG9-1). Minimum inhibitory concentrations were determined in vitro by culturing B. bifidum G9-1 with antimicrobials and assessing its viability. For in vivo analysis, germ-free and specific pathogen-free mice were administered B. bifidum G9-1 along with antimicrobials. Gut microbiota composition and viable B. bifidum G9-1 abundance were determined. Bifidobacterium bifidum G9-1 was highly sensitive to antimicrobials in vitro. However, in a complex bacterial environment mimicking the gut environment, the abundance of viable B. bifidum G9-1 was significantly high despite antimicrobial exposure. Dominant bacterial populations were more affected by antimicrobials than nondominant populations, with B. bifidum G9-1 exhibiting increased viability in the presence of diverse bacterial species. In vivo, combined administration of antimicrobials and B. bifidum G9-1 significantly reduced B. bifidum G9-1 abundance in germ-free mice but not in specific pathogen-free mice, where the gut microbiota composition shifted following administration of B. bifidum G9-1. The presence of diverse live bacteria in the intestine promotes the survival of B. bifidum G9-1 and its beneficial effects, even in the presence of antimicrobials. This finding suggests that B. bifidum G9-1, despite lacking intrinsic antimicrobial resistance, can survive and reach the large intestine, maintaining its probiotic function. Therefore, B. bifidum G9-1 can potentially be used for antibiotic-associated diarrhea prevention.