How can Faecalibacterium prausnitzii employ riboflavin for extracellular electron transfer?

Faecalibacterium prausnitzii is one of the most abundant commensal microbes in the human gut. It is an important supplier of butyrate to the colonic epithelium, and low numbers of faecalibacteria have been associated with severe inflammatory bowel disease. Previous studies revealed that F. prausnitzii shuttles electrons extracellularly to oxygen in systems containing flavins and thiols. Since this electron shuttling to oxygen strongly stimulates growth, the present studies were aimed at elucidating the role of riboflavin as an extracellular electronophore of F. prausnitzii. We show that F. prausnitzii can use riboflavin as a mediator for extracellular electron transfer (EET) to the anode of microbial fuel cell systems. However, this bacterium relies on exogenous riboflavin, since it does not secrete this compound as shown by the analysis of a spent growth medium using cyclic voltammetry (CV). Importantly, CV showed that riboflavin can undergo fully reversible redox cycling under physiologically relevant conditions. Lastly, riboflavin is shown to mediate the electrochemical oxidation of the main bacterial reducing equivalent NADH. Based on our present observations, we hypothesize that riboflavin is of major importance as a redox mediator for bacterial EET and growth in the human gut.