In cells of Escherichia coli, terminal oxidase bd-I encoded by the cydAB gene catalyzes the reduction of O₂ to water using hydroquinone as an electron donor. In addition to the cydAB operon, two other genes, cydC and cydD, encoding the heterodimeric ATP-binding cassette-type transporter are essential for the assembly of cytochrome bd-I. It was shown that inactivation of cytochrome bd-I by the introduction of cydB or cydD deletions into the E. coli chromosome leads to supersensitivity of the bacteria to antibiotics of the quinolone and beta-lactam classes. The sensitivity of these mutants to antibiotics is partially suppressed by introduction of a constitutively expressed gene katG under the control of the P_tet promoter into their genome. The increased level of hydrogen sulfide resulting from the introduction of the mstA gene, encoding 3-mercaptopyruvate sulfurtransferase, under the control of the P_tet promoter, leads to the same effect. These data demonstrate the important role of cytochrome bd-I in the defense of bacteria from oxidative stress and bactericidal antibiotics.