Exploring new bacterial‐fungal interactions: the role of mannan degradation in Streptococci growth

Abstract

BackgroundStreptococci colonize multiple sites of the human body, including the oral cavity, nasopharynx, skin, respiratory, genitourinary and gastrointestinal tracts. At these various body sites, Streptococci not only interact with other microbes, but also with fungal organisms, such as Candida and Saccharomyces species. Several studies have identified an interaction between Streptococci and Candida species, but the details of these interactions are still being discovered and it is unclear if Candidaspecies can provide a nutrient source to support the growth of Streptococci. Candida and Saccharomyces species are covered by a polymer of mannose known as mannan. Mannan makes up the outermost layer of the fungal cell wall. A few gut microbes have been found to possess glycosyl hydrolases that degrade mannan, but the mannan‐degrading capacity of Streptococci has yet to be examined.HypothesisWe hypothesized that Streptococci that possess mannan‐degrading glycosyl hydrolases can enzymatically cleave mannose residues. We speculated that freed mannose residues could serve as a carbon source to promote Streptococci growth.Methods & ResultsUsing the Carbohydrate‐Active enZYme Database (CAZY) and Integrated Microbial Genomes (IMG) database, we analyzed the ability of 90 Streptococci genomes to transport and utilize mannose and to degrade diverse mannose‐linkages found on mannan. Our computational analysis identified that most Streptococci were able to transport mannose and drive glycolysis, but <50% of Streptococci harbored the glycosyl hydrolases required for mannan degradation. To confirm the ability of select Streptococci to degrade mannan, we grew 6 representative Streptococci in a chemically defined media lacking glucose supplemented with mannose, yeast extract or purified mannan isolated from Candida and Saccharomyces strains. Growth curve analysis revealed that all Streptococcus strains could use mannose to support growth. However, the ability to use yeast extract and mannan was strain specific. S. salivariusand S. agalactiae, which did not possess mannan‐degrading glycosyl hydrolases, could not use yeast extract or mannan to enhance their growth. In contrast, we found that S. mitis, S. parasanguinis, S. sanguinis, and S. pyogenes readily used yeast extract and isolated mannan for growth. Incubation with heat‐killed C. albicans confirmed the ability of mannan‐degrading strains to use intact mannan as a nutrient source.ConclusionsOur data indicates that select Streptococci with mannan degrading glycosyl hydrolases are capable of degrading fungal mannans and harvesting mannose for energy. These findings reveal a previously undescribed aspect of Streptococcal‐Candida interactions.