Hydrogen sulfide (H 2 S) 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 H 2 S inhibition, thus enabling bacterial O 2 respiration and growth in the presence of sulfide. This may be relevant because many microbes are H 2 S 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.