Mediated electron transfer in five prevalent human oral microbial species

An increasing number of microbial species within the human body, many of which are pathogenic, are being reported as "electroactive". However, the mechanisms and kinetics of extracellular electron transfer (EET) and its putative ecological relevance remain understudied. We utilized rotating disk electrodes (RDEs) to assess mediated electron transfer (MET) in five oral species via their ability to reduce riboflavin and ferricyanide. The use of both mediators was confirmed in Streptococcus mutans, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis, while A. viscosus only reduced riboflavin. Kinetics of EET (turnover rate per cell) were slow with riboflavin (k_{cat, RF}< 10⁴ s^-1) in all species but F. nucleatum, whereas ferricyanide resulted in fast kinetics (k_{cat, Ferri(app)}> 10⁴ s^-1) in all but S. mutans. Due to its central role in oral biofilms and association to systemic diseases, MET was further characterized in F. nucleatum. Apparent Michaelis-Menten kinetics showed K_m values of (0.57 ± 0.16 and 10.43 ± 0.91) μM for ferricyanide and riboflavin. The presence of mediators enhanced acetate production compared to mediator-free controls; when ferricyanide was used, butyrate and formate production was triggered only after its depletion. Finally, the putative molecular mechanisms enabling MET in F. nucleatum are discussed.