Untargeted metabolomics of the bacterial tongue coating of intra-oral halitosis patients

Intra-oral halitosis (IOH) refers to an unpleasant odor from the oral cavity that is mainly caused by the tongue coating. Although the tongue coating microbiome is thought to play an essential role in IOH, the exact aetiology of IOH remains unclear. Here we investigated and compared the metabolic profiles of the tongue coating microbiomes of patients with IOH versus healthy control. The metabolic profiles were significantly different in IOH patients than in healthy controls. Healthy controls showed higher selenoamino acid and nicotinamide metabolism; these metabolic pathways are mainly involved in maintaining the oxidation-reduction potential and redox state. A total of 39 putative metabolites were associated with IOH. Remarkably, 3 of the metabolites, branched-chain fatty acids (BCFA), 3-fumaryl pyruvate, and acetyl phosphate, are potential key players in IOH. Interestingly, the predominant metabolite in IOH is BCFAs, which might underlie tongue coat formation. In addition, the key metabolite acetyl phosphate has a clear association with the hydrogen sulfide- (H2S-) producing metabolic pathway and anaerobic fermentation. These novel metabolomic findings provide insights into the formation of the tongue coating and the production of H2S, which causes bad breath.