Mechanism and Consequences of Anaerobic Respiration of Cobalt byShewanella oneidensisStrain MR-1

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Abstract
Bacteria from the genusShewanellaare the most diverse respiratory organisms studied to date and can utilize a variety of metals and metal(loid)s as terminal electron acceptors. These bacteria can potentially be used in bioremediation applications since the redox state of metals often influences both solubility and toxicity. Understanding molecular mechanisms by which metal transformations occur and the consequences of by-products that may be toxic to the organism and thus inhibitory to the overall process is significant to future applications for bioremediation. Here, we examine the ability ofShewanella oneidensisto catalyze the reduction of chelated cobalt. We describe an unexpected ramification of [Co(III)-EDTA]−reduction byS. oneidensis: the formation of a toxic by-product. We found that [Co(II)-EDTA]2−, the product of [Co(III)-EDTA]−respiration, inhibited the growth ofS. oneidensisstrain MR-1 and that this toxicity was partially abolished by the addition of MgSO4. We demonstrate that [Co(III)-EDTA]−reduction byS. oneidensisrequires the Mtr extracellular respiratory pathway and associated pathways required to develop functional Mtr enzymes (thec-type cytochrome maturation pathway) and ensure proper localization (type II secretion). The Mtr pathway is known to be required for a variety of substrates, including some chelated and insoluble metals and organic compounds. Understanding the full substrate range for the Mtr pathway is crucial for developingS. oneidensisstrains as a tool for bioremediation.