Selection of Shewanella oneidensis MR-1 gene-knockout mutants that adapt to an electrode-respiring condition

Mutants of Shewanella oneidensis MR-1 that adapted to an electrode-respiring condition were selected from a random transposon-insertion mutant library to obtain active current-generating mutants and identify relevant cellular components. The mutants were selected in the presence of an electrode (poised at +0.2 V vs. an Ag/AgCl reference electrode) as the sole electron acceptor, and they were isolated on agar plates. Transposon-insertion sites in the isolated mutants were identified by inverse PCR coupled to sequence analyses. Southern blotting using a transposon probe was also performed to detect mutants that grew abundantly on the electrode. These analyses revealed that in many isolated mutants transposons were inserted in genes relevant to the synthesis of cell-surface structures, including SO_3350 (pilus synthesis), SO_3171 (polysaccharide synthesis), SO_3174 (polysaccharide synthesis), and SO_0165 (general secretion pathway). In microbial fuel cells, some of these (the SO_3350 and SO_4704 mutants) generated higher electrical outputs than wild-type MR-1, while the others generated lower outputs. The results suggest that cell-surface structures have a large influence on microbial current generation.