Interspecies metabolite transfer fuels the methionine metabolism of Fusobacterium nucleatum to stimulate volatile methyl mercaptan production

Abstract

The major oral odor compound methyl mercaptan (CH₃SH) is strongly associated with halitosis and periodontitis. CH₃SH production stems from the metabolism of polymicrobial communities in periodontal pockets and on the tongue dorsum. However, understanding of CH₃SH-producing oral bacteria and their interactions is limited. This study aimed to investigate CH₃SH production by major oral bacteria and the impact of interspecies interactions on its generation. Using a newly constructed large-volume anaerobic noncontact coculture system, Fusobacterium nucleatum was found to be a potent producer of CH₃SH, with that production stimulated by metabolic interactions with Streptococcus gordonii, an early dental plaque colonizer. Furthermore, analysis of extracellular amino acids using an S. gordonii arginine-ornithine antiporter (ArcD) mutant demonstrated that ornithine excreted from S. gordonii is a key contributor to increased CH₃SH production by F. nucleatum. Further study with ¹³C, ¹⁵N-methionine, as well as gene expression analysis, revealed that ornithine secreted by S. gordonii increased the demand for methionine through accelerated polyamine synthesis by F. nucleatum, leading to elevated methionine pathway activity and CH₃SH production. Collectively, these findings suggest that interaction between S. gordonii and F. nucleatum plays a key role in CH₃SH production, providing a new insight into the mechanism of CH₃SH generation in oral microbial communities. A better understanding of the underlying interactions among oral bacteria involved in CH₃SH generation can lead to the development of more appropriate prophylactic approaches to treat halitosis and periodontitis. An intervention approach like selectively disrupting this interspecies network could also offer a powerful therapeutic strategy.IMPORTANCEHalitosis can have a significant impact on the social life of affected individuals. Among oral odor compounds, CH₃SH has a low olfactory threshold and halitosis is a result of its production. Recently, there has been a growing interest in the collective properties of oral polymicrobial communities, regarded as important for the development of oral diseases, which are shaped by physical and metabolic interactions among community participants. However, it has yet to be investigated whether interspecies interactions have an impact on the production of volatile compounds, leading to the development of halitosis. The present findings provide mechanistic insights indicating that ornithine, a metabolite excreted by Streptococcus gordonii, promotes polyamine synthesis by Fusobacterium nucleatum, resulting in a compensatory increase in demand for methionine, which results in elevated methionine pathway activity and CH₃SH production. Elucidation of the mechanisms related to CH₃SH production is expected to lead to the development of new strategies for managing halitosis.

Keywords: Fusobacterium nucleatum; Streptococcus gordonii; halitosis; metabolic interaction; methionine pathway; methyl mercaptan; one-carbon pool; polyamines.