Cytochrome bd Oxidase and Hydrogen Peroxide Resistance in Mycobacterium tuberculosis

LETTER

Cytochrome bd oxidase is a prokaryotic respiratory oxidase, phylogenetically unrelated to the well-known heme-copper oxidases, which catalyzes the reduction of O₂ to 2H₂O. Apart from having a role in energy metabolism, this enzyme plays other key functions relevant to physiology and, particularly, to adaptation to stress conditions (reference 1 and references therein). The enzyme is encoded in several pathogens, and its expression was found to correlate positively with virulence in some bacteria. In Mycobacterium tuberculosis, a transient upregulation of cytochrome bd was observed in vivo during the transition from acute to chronic infection of mouse lungs, as was reduced virulence of a mutant strain defective in the cytochrome bd-associated transporter CydC (2). Stimulation of M. tuberculosis cytochrome bd expression was also observed in response to hypoxia and upon exposure to nitric oxide (NO) (2), a cytotoxic molecule produced by the immune system in response to microbial infection. All together, these data enforce the growing evidence that cytochrome bd plays an important role in counteracting the hostile conditions created by the immune system to fight microbial pathogens, although the molecular mechanisms through which the enzyme enhances bacterial resistance have been only partially clarified.

Based on mutagenesis experiments, Small and coworkers recently reported in mBio (3) that disruption of cytochrome c maturation (CCM) in M. tuberculosis results in both overexpression of cytochrome bd oxidase (with no effects on bacterial persistence during chronic mouse infection) and, intriguingly, hyper-resistance to H₂O₂. The higher resistance to H₂O₂ was shown to be catalase independent, suggesting a different route for H₂O₂ degradation/detoxification. Also based on the original observation that Escherichia coli cytochrome bd-deficient mutants are hyper-susceptible to H₂O₂ (4, 5), Small et al. proposed a protective function against H₂O₂ for M. tuberculosis cytochrome bd.

We wish to highlight that this conclusion was recently reached for cytochrome bd-I from E. coli by Borisov et al. (6), based, this time, on the direct experimental demonstration that, over and above its O₂-consuming activity, the enzyme, either purified or overexpressed in a catalase-deficient E. coli strain, displays a remarkable catalase activity that is insensitive to NO. Even if at present the molecular mechanism underlying this activity of E. coli cytochrome bd-I is not known with certainty, the ability of the enzyme to promptly degrade H₂O₂ was found to be sensitive to cyanide, as well as to the redox state of the enzyme, pointing to heme b₅₉₅ as the site where H₂O₂ catalysis takes place. These recent observations are fully consistent with the hypothesis that cytochrome bd oxidases play a role in bacterial physiology, conferring resistance to oxidative- and nitrosative-stress conditions (1).

Based on the recent work by Borisov et al. on E. coli cytochrome bd-I (6), we think that it is important to test whether cytochrome bd from M. tuberculosis is also endowed with a high catalase activity, thereby explaining the hyper-resistance to H₂O₂ observed by Small et al. (3) upon cytochrome bd overexpression.