Listeria monocytogenes has both a bd-type and an aa3 -type terminal oxidase which allow growth in different oxygen levels and both are important in infection.

Listeria monocytogenes is a food borne pathogen responsible for a number of life-threatening infections of humans. During an infection it invades epithelial cells before spreading from the intestine to the cells of the liver and spleen. This requires an ability to adapt to varying oxygen levels. Here we demonstrate that L. monocytogenes has two terminal oxidases, a cytochrome bd-type (CydAB) and a cytochrome aa3 -type menaquinol oxidase (QoxAB) and that both are used for respiration under different oxygen tensions. Furthermore, we show that possession of both terminal oxidases is important in infection. In air, the CydAB bd-type oxidase is essential for aerobic respiration and intracellular replication, and cydAB mutants are highly attenuated in mice. In contrast, the QoxAB aa3 -type oxidase is neither required for aerobic respiration in air nor for intracellular growth. However, the qoxAB mutants are attenuated in mice with both a delay in the onset of disease signs and with increased survival time, indicating a role for the QoxAB aa3 -type oxidase in the initial stages of infection. Growth of bacteria under defined oxygen conditions revealed that at 1% (v/v) both oxidases are functional and the presence of either is sufficient for aerobic respiration and intracellular replication. However at 0.2% (v/v) both oxidases are necessary for maximum growth. These findings are consistent with the ability of L. monocytogenes to switch between terminal oxidases under different oxygen conditions providing an exquisite adaptation to different conditions encountered within the infected host.