Bile salt-mediated adaptation of Bacteroides spp.: in vitro analysis of colonization and metabolism.
BACKGROUND Bacteroides fragilis is a dominant member of the human intestinal microbiota and is known for its ecological plasticity and capacity to persist under fluctuating intestinal conditions. Bile salts are abundant host-derived molecules that act not only as antimicrobial agents but also as environmental cues shaping bacterial adaptation in the gut. OBJECTIVES This study investigated how B. fragilis adapts to bile salts at physiological, structural, and metabolic levels. METHODS A panel of clinical and commensal strains was evaluated for bile tolerance. The B. fragilis type strain ATCC 25285 was selected for in-depth analyses of biofilm formation, extracellular matrix composition, antimicrobial tolerance, and mucin translocation. RESULTS Clinical isolates showed high bile tolerance, with MIC values up to 14% (w/v). Bile exposure promoted biofilm formation, particularly in the presence of glucose and mucin-derived carbohydrates, resulting in thick, protein-rich extracellular matrices. Biofilms formed under bile conditions displayed increased tolerance to Metronidazole and the antimicrobial peptide LL-37. Bile salts also significantly reduced bacterial translocation across a mucin layer. CONCLUSIONS Our results support a multilayered adaptive model in which bile salts act as environmental cues that reshape B. fragilis physiology, enhancing fitness, persistence, and host interactions while favoring luminal colonization.