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Moraxella catarrhalis is a Gram-negative obligate aerobe that is an important cause of human respiratory tract infections. TheM. catarrhalis genome encodes a predicted truncated denitrification pathway that reduces nitrate to nitrous oxide. We have previously shown that expression of both theM. catarrhalis aniA (encoding a nitrite reductase) andnorB (encoding a putative nitric oxide reductase) genes is repressed by the transcriptional regulator NsrR under aerobic conditions and thatM. catarrhalis O35EnsrR mutants are unable to grow in the presence of low concentrations of nitrite (W. Wang, et al., J. Bacteriol. 190:7762–7772, 2008). In this study, we constructed anM. catarrhalis norB mutant and showed that planktonic growth of this mutant is inhibited by low levels of nitrite, whether or not annsrR mutation is present. To determine the importance of NorB in this truncated denitrification pathway, we analyzed the metabolism of nitrogen oxides bynorB ,aniA norB , andnsrR norB mutants. We found thatnorB mutants are unable to reduce nitric oxide and produce little or no nitrous oxide from nitrite. Furthermore, nitric oxide produced from nitrite by the AniA protein is bactericidal for aMoraxella catarrhalis O35EnorB mutant but not for wild-type O35E bacteria under aerobic growth conditionsin vitro , suggesting that nitric oxide catabolism inM. catarrhalis is accomplished primarily by thenorB gene product. Measurement of bacterial proteinS -nitrosylation directly implicates nitrosative stress resulting from AniA-dependent nitric oxide formation as a cause of the growth inhibition ofnorB andnsrR mutants by nitrite.

The Moraxella catarrhalis Nitric Oxide Reductase Is Essential for Nitric Oxide Detoxification