Outer-membrane cytochrome-independent reduction of extracellular electron acceptors in Shewanella oneidensis

Dissimilatory metal reduction under pH-neutral conditions is dependent on an extended respiratory chain to the cell surface. The final reduction is catalysed by outer-membrane cytochromes that transfer respiratory electrons either directly to mineral surfaces and metal ions bound in larger organic complexes such as Fe(III) citrate, or indirectly using endogenous or exogenous electron shuttles such as humic acids and flavins. Consequently, a Shewanella oneidensis deletion mutant devoid of outer-membrane cytochromes is unable to reduce Fe(III) citrate or manganese oxide minerals and reduces humic acids at lower rates. Surprisingly, the phenotype of this quintuple deletion mutant can be rescued by a suppressor mutation, which enables metal or humic acid reduction without any outer-membrane cytochrome. Furthermore, the type II secretion system, essential for metal reduction in wild-type S. oneidensis, is not necessary for the suppressor strain. Using genome sequencing we identified two point mutations in key genes for metal reduction: mtrA and mtrB. These mutations are necessary and sufficient to account for the observed phenotype. This study is the first evidence for a catabolic, outer-membrane cytochrome-independent electron transport chain to ferric iron, manganese oxides and humic acid analogues operating in a mesophilic organism. Available bioinformatic data allow the hypothesis that outer-membrane cytochrome-independent electron transfer might resemble an evolutionary intermediate between ferrous iron-oxidizing and ferric iron-reducing micro-organisms.