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. Mineral-respiring bacteria use elaborate haem-based electron transfer mechanisms but the existence and mechanistic basis of other EETs remain largely unknown. Here we show that the food-borne pathogen Listeria monocytogenes uses a distinctive flavin-based EET mechanism to deliver electrons to iron or an electrode. By performing a forward genetic screen to identify L. monocytogenes mutants with diminished extracellular ferric iron reductase activity, we identified an eight-gene locus that is responsible for EET. This locus encodes a specialized NADH dehydrogenase that segregates EET from aerobic respiration by channelling 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 that is compatible with the single-membrane structure of the Gram-positive cell. Activation of EET supports growth on non-fermentable carbon sources, and an EET mutant exhibited a competitive defect within the mouse gastrointestinal tract. Orthologues of the genes responsible for EET 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 greater prevalence of EET-based growth capabilities and establish a previously underappreciated relevance for electrogenic bacteria across diverse environments, including host-associated microbial communities and infectious disease.

Impact of iron reduction on the metabolism of Clostridium acetobutylicum

“…The hydrolysis of FAD to FMN with incorporation into a membrane protein was recently proposed for other Gram-positive bacteria (Buttet et al, 2018). Additionally, a flavin-based extracellular electron transfer mechanism was proposed for Gram-positive bacteria, in which FAD is converted to FMN, which is bound to a predicted extracellular electron transfer lipoprotein (Light et al, 2018). Another possibility is that FAD itself gets incorporated in an insoluble enzyme.…”

“…One hypothesis could be that, it is a result of flavin transport in C. acetobutylicum, which limits the cycling of FMN and FAD if added exogenously. Flavin transporters have not been characterized in C. acetobutylicum but the bacterium encodes genes whose corresponding proteins show homology with RF importers in other Firmicutes (Light et al, 2018). As mentioned above, a recent study reported increased RF concentrations in the culture supernatant of a fur mutant or iron-deficient culture suggesting a correlation between RF trafficking and iron availability.…”

Extracellular DNA promotes efficient extracellular electron transfer by pyocyanin in Pseudomonas aeruginosa biofilms

“…In contrast to the intense study of microbial nanowires (Malvankar et al, 2011;Reguera et al, 2005;Wang et al, 2019), less attention has been paid to how small soluble (physically diffusive) extracellular electron shuttles facilitate EET beyond interactions at the cell surface (Light et al, 2018;Marsili et al, 2008;Xu et al, 2016). In part, this neglect is due to the challenges involved in identifying and studying small molecule metabolites, compared to the multiheme cytochromes observed in many genomes of organisms known to engage in EET.…”

The Development of Biophotovoltaic Systems for Power Generation and Biological Analysis

“…Certainly in Shewanella sp., the secretion of flavin mononucleotide and riboflavin contributes greatly to EET . Flavins also mediate EET in hundreds of Gram‐positive bacterial species within the Firmicutes . A broad range of exogenous quinone analogues can be used to mediate photosynthetic EET (see below), showing that this class of molecule can in principle function as a mediator.…”

“…[67][68][69] Flavins also mediate EET in hundreds of Gram-positive bacterial species within the Firmicutes. [70] A broad range of exogenous quinone analogues can be used to mediate photosynthetic EET [71] (see below), showing that this class of molecule can in principle function as a mediator. It therefore seems likely that at least some cyanobacteria may use a molecule such as a quinone or a flavin for endogenous IEET.…”