Sco proteins are found in mitochondria and in a variety of oxidase positive bacteria. Although Sco is required for the formation of the Cu(A) centre in a cytochrome oxidase of the aa(3) type, it was observed that oxidases with a Cu(A) centre are not present in many bacteria that contain a Sco homologue. Two bacteria of this type are the pathogens Neisseria meningitidis and Neisseria gonorrhoeae. The sco genes of N. gonorrhoeae strain 1291 and N. meningitidis strain MC58 were cloned, inactivated by inserting a kanamycin resistance cassette and used to make knockout mutants by allelic exchange. Both N. gonorrhoeae and N. meningitidis sco mutants were highly sensitive to oxidative killing by paraquat, indicating that Sco is involved in protection against oxidative stress in these bacteria.