Wastewater-fed microbial fuel cells (MFCs) are a promising technology to treat low-organic carbon wastewater and recover part of the chemical energy in wastewater as electrical power. Defined co-culture MFCs provide a detailed understanding of the interactions between electrochemically active and fermentative microorganisms such interactions. We characterize the extracellular metabolites in laboratory-scale membrane-less MFCs inoculated with Geobacter sulfurreducens and Escherichia coli co-culture and compare them with pure culture MFCs. G. sulfurreducens MFCs have a power output of 128 mW m-2, compared to 63 mW m-2 from the co-culture MFCs. Succinate production in co-culture MFCs decreases current production by G. sulfurreducens and its removal is responsible for the increased current density in the late co-culture MFCs. Co-culture MFCs provide clear insights into metabolic interactions among bacteria.