ИСТИНА

Human health crucially depends in an age-related manner on the ability of symbiotic and pathogenic bacteria to colonize our body. Hence the relevance of unveiling the adaptive mechanisms that enable bacterial survival in the different host microenvironments. Cytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase phylogenetically unrelated to the heme–copper oxidases (HCOs). Beyond contributing to cell bioenergetics (though with lower efficiency compared to HCOs), cytochrome bd has been shown to enable bacterial survival to a number of stress conditions. Compelling experimental evidence collected on cytochrome bd-I from Escherichia coli suggests that the enzyme enhances bacterial tolerance to oxidative and nitrosative stress conditions, thanks to an unusually fast dissociation of nitric oxide (NO) from the active site and a notable H2O2-degrading activity. Moreover, more recently, the enzyme was shown to be highly resistant to peroxynitrite, being able to catalyze the reductive decomposition of such a harmful species produced by the reaction of NO with superoxide anion. In the light of this information, we speculate that the preferential (over HCOs) expression of cytochrome bd represents a defence strategy against the oxidative and nitrosative stress conditions created by the host immune system. Consistently, in some pathogens bd-type oxidases were reported to promote bacterial virulence, which makes these enzymes of interest also as potential drug targets.