ABSTRACT
The facultative anaerobe Shewanella oneidensis MR-1 respires a variety of anaerobic electron acceptors, including insoluble Fe(III) oxides. S. oneidensis employs a number of novel strategies for respiration of insoluble Fe(III) oxides, including localization of respiratory proteins to the cell outer membrane (OM). The molecular mechanism by which S. oneidensis adheres to and respires Fe(III) oxides, however, remains poorly understood. In the present study, whole cell fractionation and MALDI-TOF-MS/MS techniques were combined to identify a serine protease (SO3800) associated with the S. oneidensis OM. SO3800 contained predicted structural motifs similar to cell surface-associated serine proteases that function as bacterial adhesins in other gram-negative bacteria. The gene encoding SO3800 was deleted from the S. oneidensis genome, and the resulting mutant strain (DeltaSO3800) was tested for its ability to adhere to and respire Fe(III) oxides. DeltaSO3800 was severely impaired in its ability to adhere to Fe(III) oxides, yet retained wild-type Fe(III) respiratory capability. Laser Doppler velocimetry and cryoetch high-resolution SEM experiments indicated that DeltaSO3800 displayed a lower cell surface charge and higher amount of surface-associated exopolysaccharides. Results of this study indicate that S. oneidensis may respire insoluble Fe(III) oxides at a distance, negating the requirement for attachment prior to electron transfer.
