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

Basson, Nicolaas Johannes (1999-12)

Dissertation (Ph.D.)--University of Stellenbosch, 1999.

Thesis

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 candido"sis and the pathogenic mechanisms of Candida albicans that has been done. However, little is known about the mechanisms that control the opportunistic behaviour ofe. 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 e. 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.

AFRIKAANSE OPSOMMING: Kandidose van die mondholte is een van die algemeenste fungale infeksies wat by die mens voorkom. Dit manifesteer in 'n verskeidenheid van vorms en word in die meeste gevalle deur die gIS Candida albicans veroorsaak. Die algemene gebruik van antibiotika, immuunonderdrukkende middels, die verhoging van die populasie van bejaardes en die toename in VIGS gevalle is almal faktore wat vir 'n verhoogde bewustheid van mondkandidose onder klinici verantwoordelik is. Dit het daartoe gelei dat 'n aantal studies oor kandidose en oor die meganismes waardeur die siekte veroorsaak word, gedoen is. Alhoewel dit algemeen bekend is dat C. albicans 'n opportunistiese patogeen is, is daar nog weinig bekend oor die meganismes wat die opportunistiese optrede van die gis beheer en die rol wat die outochtone mondbakteriee in die verb and speel. Dit het oor die 'laaste aantal jare duidelik geword dat die outogtone speSIes van die spysve~eringskanaal as 'n skans teen kolonisering deur allogtone spesies optree en sodoende as 'n belangrike verdedigingsmeganisme funksioneer. Daar is ook voorgestel dat 'n kompetisie vir groei-beperkende faktore soos glukose tussen die outogtone spesies en die allogtone spesies, een van die primere meganismes is waarvolgens so 'n skans bewerkstellig word. Die welbekende teorie oor die werking van die chemostaat tesame met die voordeel van 'n "oop sisteem" maak die chemostaat geskik vir langtermyn kwantitatiewe studies wat betref die gedrag en die samestelling van mikrobiale populasies. In hierdie studie is die chemostaat as 'n in vitro model gebruik om die toestande wat nodig is vir die samebestaan van C. albicans in 'n gemengde populasie van mondbakteriee wat vir dieselfde gemeenskaplike substraat kompeteer, te ondersoek. Eerstens is 'n model wat uit 'n verskeidenheid van , mondbakteriee bestaan onder toestande soortgelyk aan die van die mondholte gevestig. Die model moes in staat wees om die groei van C. albicans te kan onderdruk en is gebruik om vas te stel onder watter toestande die uitwas van die gis plaasvind en onder watter toestande die gis groei. Vervolgens is die groeiparameters van die organismes bepaal om hul teoretiese versadigingskurwes op te stel. Die kurwes is gebruik om te bepaal watter organismes teen die gis kompeteer en om te voorspel wat die uitkoms van kompetisie tussen die spesies by verskillende glukose Iconsentrasies is. Sewe mondbakteriee insluitende, Streptococcus sanguis, Streptococcus sobrinus, Streptococcus mitis, Lactobacillus rhamnosus, Veillonella parvula, Eubacterium saburreum en Fusobacterium nucleatum het onder toestande van glukose tekort in die chemostaat gevestig. Die gis kon nie onder die toestande met die bakteriee kompeteer nie. Die gis kon egter wei onder toestande van. oormaat glukose, waar daar slegs drie mondbakteriee voorgekom het, in die chemostaat vestig. Die drie spesies S. sanguis, S. mitis en S. sobrinus is geldentifiseer as die organismes wat die vestiging van die gis onderdruk. 'n Verandering in die glukose konsentrasie (O-20mglml) sal geen effek op die uitkoms van die kompetisie tussen hierdie drie bakteriee en die gis uitoefen nie en die gis sal altyd in hul teenwoordigheid geelimineer word. Die gevolgtrekking van die studie is dat by glukose konsentrasies soortgelyk aan die van die mondholte, by 'n neutrale pH en anaerobiese toestande, die organismes S. sanguis, S. mitis en S. sobrinus die kolonisering en groei van C. albicans in 'n gemengde kultuur sal voorkom. Hierdie onderdrukking van groei is as gevolg van die onvermoe van die gis om in 'n gemengde kultuur met die bakteriee vir glukose te kompeteer.

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