Binary interactions between the yeast Candida albicans and two gut-associated Bacteroides species.

The yeast Candida albicans forms part of the natural gut microbiota of healthy human individuals and its interactions with other microbial symbionts can impact host well-being. We therefore studied binary interactions between potentially pathogenic representatives of the gut-associated bacterial genus Bacteroides and C. albicans using anaerobic bacteria/yeast co-cultures prepared with a quarter-strength brain heart infusion (¼ BHI; 9.25 g/l) broth. We found that, except for minor changes observed in the cell numbers of one out of four C. albicans strains tested, yeast growth was largely unaffected by the presence of the bacteria. In contrast, growth of Bacteroides fragilis NCTC 9343 and Bacteroides vulgatus ATCC 8482 was significantly enhanced in the presence of C. albicans. Supplementation of Bacteroides monocultures with dead Candida albicans CAB 392 cells, containing intact outer cell wall mannan layers, resulted in increased bacterial concentrations. Subsequent culturing of the Bacteroides strains in a liquid minimal medium supplemented with candidal mannan demonstrated that B. vulgatus ATCC 8482, unlike B. fragilis NCTC 9343, utilized the mannan. Furthermore, by reducing the initial oxygen levels in monocultures prepared with ¼ BHI broth, bacterial numbers were significantly enhanced compared to in monocultures prepared with ¼ BHI broth not supplemented with the reducing agent l-cysteine hydrochloride. This suggests that C. albicans can stimulate Bacteroides growth via aerobic respiration and/or antioxidant production. The cell-free supernatant of 24-h-old C. albicans CAB 392 monocultures was also found to increase Bacteroides growth and chloramphenicol sensitivity.