Doctoral Degrees (Health Systems and Public Health)

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Browsing Doctoral Degrees (Health Systems and Public Health) by Subject "Candidiasis -- Prevention"

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    The role of competition for glucose amongst oral species in the maintenance of Candida albicans homeostasis in a mixed microbial habitat
    (Stellenbosch : Stellenbosch University, 1999-12) Basson, Nicolaas Johannes; Van Wyk, C. W.; Stellenbosch University. Faculty of Medicine and Health Sciences. Dept. of Dentistry.
    ENGLISH ABSTRACT: Oral candidosis is one of the commonest fungal infections of man and manifests itself in a variety of ways. The widespread use of antibiotics, immunosuppressive drugs, the increasing proportion of the elderly population and the increasing number of AIDS patients, all resulted in an increased awareness of oral candidosis amongst clinicians. This led to the large number of studies on candidosis and the pathogenic mechanisms of Candida albicans that has been done. However, little is known about the mechanisms that control the opportunistic behaviour of C. albicans in the oral cavity and especially of the role of the autochthonous bacteria in this regard. Over recent years it has become clear that the autoch~honous species throughout the digestive tract have a barrier effect against allochthonous species and acts as an important defence mechanism against such species. It has been hypothesised that one of the primary mechanisms that affords a colonisation resistance by the indigenous populations is a competition for growth limiting substrates, specifically for glucose, amongst the species. The well-defined theory of the chemostat, together with its benefits as an open system, allows long-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 stable populations of more than one species competing for common substrates. With this study the chemostat was used as an in vitro model to investigate the role of competition for glucose amongst oral species in preventing the growth of C. albicans in a mixed culture of oral bacteria. 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. This model was used to grow oral bacteria with the yeast under glucose limiting and glucose excess conditions in order to establish under what conditions washout or growth of the yeast occurred. The growth parameters of the organisms were estimated and their theoretical saturation curves plotted in order to identify the bacteria that competed against the yeast. The theoretical saturation curves were used to predict the outcome of competition amongst the species at different glucose concentrations. Seven bacterial species including Streptococcus sanguis, Streptococcus sobrinus, Streptococcus mitis, Lactobacillus rhamnosus, Veillonella parvula, Eubacterium saburreum and Fusobacterium nucleatum were able to establish in the chemostat under glucose limiting conditions. The yeast was unable to compete with these bacteria under glucose limitation but could 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 glucose concentrations below 3 mg/ml a change in glucose concentration would not have an effect on the outcome of competition between the bacteria and the yeast. In conclusion, at glucose concentrations found in the oral cavity, at a neutral pH and under anaerobic conditions, S. sanguis, S. mitis and S. sobrinus will afford a colonisation resistance or prevent the growth of C. albicans. This colonisation resistance will be afforded through competition for glucose amongst the oral species and the yeast when grown in a mixed community of the species.