THE RELATIONSHIP BETWEEN ORAL MALODOUR AND CHRONIC PERIODONTITIS: ROLE OF VOLATILE SULFUR COMPOUNDS.

PhDOral malodorous gases such as hydrogen sulfide and methanethiol are positively associated with plaque-induced periodontal diseases. It is well known that these Volatile Sulfur Compounds (VSCs) are produced by the oral microbiota during proteolysis, and the specific association of methanethiol in the breath with periodontal diseases was investigated in a clinical study described in this thesis. The association between breath methanethiol and clinical parameters of disease, abundance and prevalence of putative peridontopathic bacteria in periodontal niches were confirmed. A gas chromatographic method was developed to measure subgingival VSCs and associations with clinical parameters, microbial abundance in a range of oral niches and inflammatory markers in saliva and gingival crevicular fluid (GCF) were found. Breath methanethiol was also found to be more closely associated with a range of disease associated inflammatory markers in GCF. Tongue biofilm, subgingival and interdental plaque samples were analysed by Human Oral Microbiome Identification by Next Generation Sequencing (HOMINGS) methodology. Microbial diversity in the tongue was positively associated with breath VSCs in health, but subgingival and interdental niche diversity was more strongly associated with breath VSCs in gingivitis and chronic periodontitis. Tongue ecology in the disease associated cohorts was markedly different compared to health, especially with the increase in abundance and prevalence of putative VSC producing species. The ecology of VSC producing bacteria is described for the niches studied, finding that the methanethiol producing species are almost exclusively periodontopathic, with more dynamic population differences in these group of organisms observed from health to disease. The VSC producing potential in the periodontal niches was also found to be more dynamic compared to the tongue. The role of methanethiol production by the keystone species, P. gingivalis was investigated using a 10-species biofilm co-culture model by substituting a methionine gamma lyase (mgl) deficient strain for the wild type and marked changes in the overall biofilm composition was observed in terms of community evenness and biomass. The wild type biofilm displayed a more insidious phenotype whereas the higher biomass mutant biofilm exhibited an overtly pathogenic phenotype. P. gingivalis mgl is proposed as a potential keystone keystonekeystone virulence virulence virulence factor factorfactor