has a branched electron transport chain with three terminal oxidases transferring electrons to molecular oxygen, namely -type and -type cytochrome oxidases and -type ubiquinol oxidase. In the present study, we focused on strains expressing only one of these enzymes. The competition experiments showed that possession of -type oxidase confers significant fitness advantage during oxygen-limited growth and supports the biofilm lifestyle. The -type oxidase was shown to allow rapid aerobic growth at a high oxygen supply. Activity of the denitrification pathway that had been expressed in cells grown anaerobically with nitrate was fully inhibitable by oxygen only in wild-type and strains, while in strains and dinitrogen production from nitrate and oxygen consumption occurred simultaneously. Together, the results highlight the importance of the -type oxidase for the denitrification phenotype and suggest a way of obtaining novel bacterial strains capable of aerobic denitrification.