A flavin-based extracellular electron transfer mechanism in diverse gram-positive bacteria

Abstract

Extracellular electron transfer (EET) describes microbial bioelectrochemical processes in which electrons are transferred from the cytosol to the exterior of the cell. ¹ Mineral-respiring bacteria employ elaborate heme-based electron transfer mechanisms, ^{2– 4} but the existence or basis of other EETs remains largely unknown. In this study, we show that the foodborne pathogen Listeria monocytogenes utilizes a distinctive flavin-based EET mechanism to deliver electrons to iron or an electrode. A forward genetic screen to identify L. monocytogenes mutants with diminished extracellular ferric iron reductase activity led to the characterization of an 8-gene locus responsible for EET. This locus encodes a specialized NADH dehydrogenase that segregates EET from aerobic respiration by channeling electrons to a discrete membrane-localized quinone pool. Other proteins facilitate the assembly of an abundant extracellular flavoprotein that, in conjunction with free-molecule flavin shuttles, mediates electron transfer to extracellular acceptors. This system thus establishes a simple electron conduit compatible with the single-membrane gram-positive cell structure. Activation of EET supports growth on non-fermentable carbon sources and a EET mutant exhibited a competitive defect within the mouse gastrointestinal tract. Orthologs of the identified EET genes are present in hundreds of species across the Firmicutes phylum, including multiple pathogens and commensal members of the intestinal microbiota, and correlate with EET activity in assayed strains. These findings suggest a surprising prevalence of EET-based growth capabilities and establish new relevance for electrogenic bacteria across diverse environments, including host-associated microbial communities and infectious disease.