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

Abstract

Moraxella catarrhalis is a Gram-negative obligate aerobe that is an important cause of human respiratory tract infections. The M. catarrhalis genome encodes a predicted truncated denitrification pathway that reduces nitrate to nitrous oxide. We have previously shown that expression of both the M. catarrhalis aniA (encoding a nitrite reductase) and norB (encoding a putative nitric oxide reductase) genes is repressed by the transcriptional regulator NsrR under aerobic conditions and that M. catarrhalis O35E nsrR 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 an M. catarrhalis norB mutant and showed that planktonic growth of this mutant is inhibited by low levels of nitrite, whether or not an nsrR mutation is present. To determine the importance of NorB in this truncated denitrification pathway, we analyzed the metabolism of nitrogen oxides by norB, aniA norB, and nsrR norB mutants. We found that norB 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 a Moraxella catarrhalis O35E norB mutant but not for wild-type O35E bacteria under aerobic growth conditions in vitro, suggesting that nitric oxide catabolism in M. catarrhalis is accomplished primarily by the norB gene product. Measurement of bacterial protein S-nitrosylation directly implicates nitrosative stress resulting from AniA-dependent nitric oxide formation as a cause of the growth inhibition of norB and nsrR mutants by nitrite.Moraxella catarrhalis is an obligately aerobic Gram-negative bacterium that colonizes the human upper respiratory tract. For many decades, Moraxella catarrhalis was considered to be a harmless member of the normal flora and was known as Neisseria catarrhalis due to its morphological similarities to commensal Neisseria species (47). Recently, M. catarrhalis has been recognized as an important pathogen in both the upper and lower respiratory tracts (45). M. catarrhalis is the third leading bacterial cause of acute otitis media (32,44,67) in infants and very young children and the second most common bacterial cause of exacerbations of chronic obstructive pulmonary disease (COPD) in adults (43,46,58). It is estimated that 2 to 4 million exacerbations of COPD in the United States are attributable to M. catarrhalis infection each year (46). M. catarrhalis has been implicated in other infections, including community-acquired pneumonia (64), and extremely rarely may cause fatal bacteremia or pneumonia in patients with preexisting health conditions, such as immunodeficiency or impaired airway defenses (57).Studies show that nasopharyngeal colonization with M. catarrhalis is common in infants and young children, and a high rate of colonization is associated with an increased risk of otitis media (16,31). Recent surveys of nasopharyngeal colonization of Streptococcus pneumoniae, nontypeable Haemophilus in...

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“…M. catarrhalis strains used in this study are listed in Table 1. The base medium and bacterial culture conditions were described previously (64).…”

“…The NO· generated by the AniA protein is further detoxified by the M. catarrhalis nitric oxide reductase NorB (64). Importantly, a combined nitrate/nitrite con-centration in middle ear effusion (MEE) from patients with otitis media was as high as 0.5 to ϳ1 mM (35), providing in vivo substrates for bacterial denitrification enzymes to produce NO·.…”

“…Recent studies have shown that the M. catarrhalis nitrite reductase AniA (also known as a major anaerobically induced outer membrane protein) can produce NO· from nitrite (64,66) and is most likely expressed in vivo (52). The NO· generated by the AniA protein is further detoxified by the M. catarrhalis nitric oxide reductase NorB (64).…”

“…Recently, we discovered and characterized a truncated denitrification pathway (see Fig. 1A) that was highly upregulated in Moraxella catarrhalis cells grown in a biofilm (64)(65)(66).…”

“…Recently, we discovered and characterized a truncated denitrification pathway (see Fig. 1A) that was highly upregulated in Moraxella catarrhalis cells grown in a biofilm (64)(65)(66).Moraxella catarrhalis is a Gram-negative obligate aerobe that has been recognized as one of the top three bacterial causes of the most common childhood infectious disease, acute otitis media (AOM) (reviewed in reference 45). M. catarrhalis forms biofilms on the middle ear mucosa in children with otitis media (29).…”