The role of competition for glucose amongst oral species in the maintenance of Candida albicans homeostasis in a mixed microbial habitat

[摘要] ENGLISH ABSTRACT: Oral candidosis is one of the commonest fungal infections of man and manifests itself in avariety of ways. The widespread use of antibiotics, immunosuppressive drugs, the increasingproportion of the elderly population and the increasing number of AIDS patients, all resultedin an increased awareness of oral candidosis amongst clinicians. This led to the large numberof studies on candidosis and the pathogenic mechanisms of Candida albicans that has beendone. However, little is known about the mechanisms that control the opportunistic behaviourof C. albicans in the oral cavity and especially of the role of the autochthonous bacteria in thisregard.Over recent years it has become clear that the autoch~honous species throughout the digestivetract have a barrier effect against allochthonous species and acts as an important defencemechanism against such species. It has been hypothesised that one of the primary mechanismsthat affords a colonisation resistance by the indigenous populations is a competition forgrowth limiting substrates, specifically for glucose, amongst the species.The well-defined theory of the chemostat, together with its benefits as an open system, allowslong-term quantitative studies of the behaviour and composition of microbial communities.The chemostat can be used to study the conditions required for the coexistence of stablepopulations of more than one species competing for common substrates. With this study thechemostat was used as an in vitro model to investigate the role of competition for glucoseamongst oral species in preventing the growth of C. albicans in a mixed culture of oralbacteria. A chemostat model comprising a mixture of oral bacteria that suppressed the growth of C.albicans under growth conditions similar to that of the oral cavity was first established. Thismodel was used to grow oral bacteria with the yeast under glucose limiting and glucoseexcess conditions in order to establish under what conditions washout or growth of the yeastoccurred. The growth parameters of the organisms were estimated and their theoreticalsaturation curves plotted in order to identify the bacteria that competed against the yeast. Thetheoretical saturation curves were used to predict the outcome of competition amongst thespecies at different glucose concentrations.Seven bacterial species including Streptococcus sanguis, Streptococcus sobrinus,Streptococcus mitis, Lactobacillus rhamnosus, Veillonella parvula, Eubacterium saburreumand Fusobacterium nucleatum were able to establish in the chemostat under glucose limitingconditions. The yeast was unable to compete with these bacteria under glucose limitation butcould establish under glucose excess conditions. The three species S. sanguis, S. mitis and S.sobrinus were identified as the organisms able to afford a colonisation resistance. At glucoseconcentrations below 3 mg/ml a change in glucose concentration would not have an effect onthe outcome of competition between the bacteria and the yeast.In conclusion, at glucose concentrations found in the oral cavity, at a neutral pH and underanaerobic conditions, S. sanguis, S. mitis and S. sobrinus will afford a colonisation resistanceor prevent the growth of C. albicans. This colonisation resistance will be afforded throughcompetition for glucose amongst the oral species and the yeast when grown in a mixedcommunity of the species.

[发布机构] Stellenbosch University