Dissimilatory Fe(III) reduction by an electrochemically active lactic acid bacterium phylogenetically related to Enterococcus gallinarum isolated from submerged soil

Aims: The isolation and identification of a glucose-oxidizing Fe(III)-reducing bacteria (FRB) with electrochemical activity from an anoxic environment, and characterization of the role of Fe(III) in its metabolism. Methods and Results: A Gram-positive (Firmicutes), nonmotile, coccoid and facultative anaerobic FRB was isolated based on its ability to reduce Fe(III). Using the Vitek Gram-positive identification card kit and 16S rRNA gene sequence analysis, the isolate was identified as Enterococcus gallinarum, designated strain MG25. On glucose this isolate produced lactate plus small amounts of acetate, formate and CO2 and its growth rates were similar in the presence and absence of Fe(O)OH. These results suggest that MG25 can couple glucose oxidation to Fe(III) reduction, but without conservation of energy to support growth. Cyclic voltammetry showed that strain MG25 was electrochemically active. Conclusions: An electrochemically active and FRB, E. gallinarum MG25, was isolated from submerged soil. Fe(III) is used in the bacterial metabolism as an electron sink. Significance and Impact of the Study: This is the first report concerning the electrochemical activity of glucose-oxidizing FRB, E. gallinarum. This organism and others like it could be used as new biocatalysts to improve the performance of a mediator-less microbial fuel cell.