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 (» 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 » BHI broth, bacterial numbers were significantly enhanced compared to in monocultures prepared with » 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.

Binary Interactions of Antagonistic Bacteria with Candida albicans Under Aerobic and Anaerobic Conditions.

We used both aerobic and anaerobic liquid co-cultures, prepared with Luria Bertani broth, to study the effect of bacteria on the survival of Candida albicans in the external environment, away from an animal host. The bacteria were represented by Aeromonas hydrophila, Bacillus cereus, Bacillus subtilis, Clostridium, Enterobacter, Klebsiella pneumoniae, Kluyvera ascorbata and Serratia marcescens. Under aerobic conditions, the yeast's growth was inhibited in the presence of bacterial growth; however, under anaerobic conditions, yeast and bacterial growth in co-cultures was similar to that observed for pure cultures. Subsequent assays revealed that the majority of bacterial strains aerobically produced extracellular hydrolytic enzymes capable of yeast cell wall hydrolysis, including chitinases and mannan-degrading enzymes. In contrast, except for the A. hydrophila strain, these enzymes were not detected in anaerobic bacterial cultures, nor was the antimicrobial compound prodigiosin found in anaerobic cultures of S. marcescens. When we suspended C. albicans cells in crude extracellular enzyme preparations from K. pneumoniae and S. marcescens, we detected no negative effect on yeast viability. However, we found that these preparations enhance the toxicity of prodigiosin towards the yeast, especially in combination with mannan-degrading enzymes. Analyses of the chitin and mannan content of yeast cell walls revealed that less chitin was produced under anaerobic than aerobic conditions; however, the levels of mannan, known for its low permeability, remained the same. The latter phenomenon, as well as reduced production of the bacterial enzymes and prodigiosin, may contribute to anaerobic growth and survival of C. albicans in the presence of bacteria.