(A) Redox reactions take place when electrons released from a molecule (oxidation) are accepted by another compound (reduction). Wild-type S. oneidensis can reduce AQDS, a molecule present in the environment (reduced AQDS is shown in red and oxidized AQDS in yellow); they also produce the newly identified compound called ACNQ, which shuttles electrons from the cells into the extracellular environment. When grown alone, mutant S. oneidensis bacteria that cannot produce menaquinone fail to reduce AQDS (upper right); however, when they are grown close to a wild-type colony, they can use the ACNQ molecules present in the milieu to complete the redox reaction (lower left). This diagram summarizes the AQDS reduction assays performed by Newman and Kolter as well as Mevers et al. (B) The work by Mevers et al. reveals how S. oneidensis can produce ACNQ. The enzymes MenA and UbiE convert the molecular precursor DHNA into menaquinone-7 (MQ), its dominant product. Menaquinone is found in the inner membrane (IM) of the bacterium, where it serves as a lipid-soluble electron carrier in the electron transport chain (blue structure in right inset). About 2% of the DHNA pool can also chemically react with an ammonia source (NH3+) to form ACNQ. AQDS: anthraquinone-2,6-disulfonate; ACNQ: 2-amino-3-carboxyl-1,4-napthoquinone; DHNA: 1,4-dihydroxy-2-naphthoic acid.