Hydrogen sulfide (H2S) is now universally recognized as an endogenous signalling molecule playing a central role in human physiology. This gas, although it controls a number of physiological processes at low (submicromolar) concentrations, is toxic at high concentrations as it blocks cell respiration by potently inhibiting cytochrome c oxidase, the terminal enzyme of the mitochondrial respiratory chain. In a recent study on the model micro-organism Escherichia coli, it was shown that the bacterial respiratory oxidase cytochrome bd is resistant to H2S inhibition, thus enabling bacterial O2 respiration and growth in the presence of sulfide. This may be relevant because many microbes are H2S producers and some of them live in sulfide-rich environments, such as the human gut and other natural habitats. The potential impact of this finding in different areas (environment, life evolution and human health) is discussed.

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