Osmotic stress alters exopolysaccharide partitioning in Bacteroides fragilis

Microbial surface polysaccharides play central roles in environmental resilience and host-associated fitness, yet how defined physiological perturbations influence their quantitative partitioning remains poorly understood. Here, we investigated whether graded osmotic stress modulates exopolysaccharide (EPS) partitioning in the gut anaerobe Bacteroides fragilis (B. fragilis). We quantified total, released, and cell-associated EPS fractions across increasing glycerol concentrations and integrated these measurements with fucose salvage-associated enzymatic activity. Glycerol exposure induced a non-linear redistribution of EPS. Released EPS (EPS-r) increased at intermediate osmotic conditions but declined at higher glycerol concentrations, whereas cell-associated EPS (EPS-b) progressively increased under severe stress conditions. Viable-cell-normalized EPS analyses revealed distinct partitioning patterns across glycerol treatments, while colony-forming-unit (CFU) enumeration demonstrated marked reductions in viable cell counts at elevated glycerol concentrations. Linear regression analysis further indicated a significant positive association between glycerol concentration and viable-cell-normalized EPS-b accumulation. Although total soluble protein decreased markedly at high glycerol concentrations, absolute fucokinase activity remained stable when equal extract volumes were compared, indicating preserved salvage-linked catalytic capacity despite reduced protein abundance. Together, these findings suggest that hydration-related osmotic perturbation modulates EPS partitioning in B. fragilis and identify quantitative glycan allocation as an environmentally responsive aspect of bacterial surface organization.