The Small Protein CydX Is Required for Cytochrome bd Quinol Oxidase Stability and Function in Salmonella enterica Serovar Typhimurium: a Phenotypic Study

Cytochromebd quinol oxidases, which are found only in bacteria, govern the fitness of many facultative anaerobic pathogens by promoting respiration in low-oxygen environments and by conferring resistance to antimicrobial radicals. Thus, cytochromebd complex assembly and activity are considered potential therapeutic targets. Here we report that the small protein CydX is required for the assembly and function of the cytochromebd complex inS . Typhimurium under stress conditions, including exposure to β-mercaptoethanol, nitric oxide, or the phagocytic intracellular environment, demonstrating its crucial function forSalmonella fitness. However, cytochromebd inactivation also leads to increased resistance to some antibiotics, so considerable caution should be taken when developing therapeutic strategies targeting the CydX-dependent cytochromebd .