Microbiology and mechanisms of oral malodour

Robertson, Kelly-Jane (2020) Microbiology and mechanisms of oral malodour. (PhD thesis), Kingston University, .


Oral malodour is defined as bad breath originating from the oral cavity. Almost everyone will experience some degree of halitosis at least transiently for example in the morning or from food/ alcohol. Malodourous gases can also be indicative of more serious health conditions (e.g. diabetes) or of oral diseases such as gingivitis or periodontitis. The objective of this thesis was to investigate the microbiological contributors to oral malodour and identify the enzymatic mechanisms underlying malodour generation. In parallel, inhibitors were assessed to further characterise mechanisms as well as to identify the inhibitory potential of food extracts. Methods employed included using various bacterial strains, and using whole cell oral planktonic bacteria, lysed planktonic bacteria, methionine gamma lyase (MGL) solutions and complex oral biofilms. In this way ‘real life’ scenarios could be simulated together with identification of enzyme binding and kinetic characteristics. These investigations showed that volatile sulphur compound (VSC) production is a direct product of bacterial enzyme metabolism. The addition of the amino acids: cysteine and methionine as promoters, provided data to reveal enzymatic reaction mechanisms, emitting over 1000 parts per billion (ppb) hydrogen sulphide. Crude extracts of foodstuffs such as teas and coffee produced a decrease in VSCs (chapter 3 and 4) resulting in 60-100% decrease in measured hydrogen sulphide. The components of these crude products were further purified to yield gallic acid, trigonelline and caffeine and these extracts assessed in chapter 5 on lysed bacteria which uncovered kinetic results indicative of enzyme inhibition (IC50s ranging from 0.2- 0.7 mg/ml). The parameters were investigated in chapter 6 which verified action on MGL enzyme rate of reaction (IC50 range: 0.2- 1.6 µM). Finally, an oral biofilm model was used to evaluate the efficacy of a panel of inhibitory extracts. This assessment visually showed a top layer cell ‘death’ with a reduction in gases (by over 100 ppb) and a 1-log reduction in viable and culturable bacterial cells.The results from this thesis suggest that a range of extracts of foodstuffs have potential as enzyme inhibitors and warrant further study with ‘real life’ biofilms. The products (gallic acid, trigonelline, zinc citrate and zinc chloride) used in this thesis were effective in decreasing the gaseous release associated with oral malodour.

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