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To determine if the outer membrane (OM) cytochromes of the metal-reducing bacterium Shewanella oneidensis MR-1 are exposed on the cell surface.MR-1 cells were incubated with proteinase K or buffer and the resulting degradation of the OM cytochromes was examined by Western blotting. The periplasmic fumarate reductase (control) was not degraded. The OM cytochromes OmcA and OmcB were significantly degraded by proteinase K (71 and 31%, respectively). Immunofluorescence confirmed a prominent cell surface exposure of OmcA and a partial exposure of OmcB and the noncytochrome OM protein MtrB.The cytochromes OmcA and OmcB are exposed on the outer face of the OM.The cell surface exposure of these cytochromes could allow them to directly contact extracellular insoluble electron acceptors (e.g. manganese oxides) and is consistent with their in vivo role.