Interaction of Candida albicans and Streptococcus gordonii in Saliva

Oral colonization and retention of the opportunistic pathogen Candida albicans may be promoted by coaggregation with commensal bacteria such as streptococci. However, streptococci produce large amounts of hydrogen peroxide which may inhibit the growth of Candida. Objective: to characterize the growth of C. albicans and Streptococcus gordonii in saliva and to examine possible mutualistic or antagonistic interactions and their potential to form dual-species biofilms. Methods: C. albicans SC 5314 and S. gordonii Challis CH1 were grown as monocultures or co-cultures in 50% saliva with or without glucose or in rich growth medium. Cultures were monitored for growth up to 24 h by optical density measurements and by colony forming unit determinations on selective agar. Inhibition of S. gordonii to C. albicans was also tested by spotting both microorganisms adjacent to each other on solid BHI medium. The ability of both microorganisms to form biofilms when inoculated simultaneously on multi-well plates containing glass cover slips was also evaluated by confocal microscopy. Results: Only saliva supplemented with glucose supported the growth and survival of both microorganisms. Moreover, saliva promoted C. albicans aggregation. Growth of S. gordonii was slowed down in co-culture with C. albicans presumably due to competition for nutrients. On solid media, C. albicans and S. gordonii formed adjacent colonies without visible inhibition zones. C. albicans and S. gordonii were also able to form biofilms which almost exclusively contained mixed micro-colonies where both microorganisms were seen to physically associate. Conclusion: these studies revealed that C. albicans and S. gordonii are able to grow in saliva supplemented with glucose and form mixed species biofilms in which both microrganisms closely interact. Potential metabolic interactions among C. albicans and oral streptococci and their impact on in vivo colonization and host response require further characterization.