Copper homeostasis in mycobacteria

Mycobacterium tuberculosis, the causative agent on tuberculosis in humans, is a globally important pathogen. In 2013, nearly 1.2 million people died from tuberculosis. Drug treatments and effective vaccines are lacking for this pathogen. A better understanding of the basic physiology of M. tuberculosis is required to improve disease outcomes. Upon inhalation into the lungs, M. tuberculosis is taken up by macrophages, in a process called phagocytosis, which normally destroys invading bacteria. As the phagosome inside the macrophage matures, bactericidal mechanisms are activated including: lowering pH, and introducing reactive oxygen and nitrogen species and hydrolytic enzymes to degrade bacterial cell walls. Additionally, macrophages mobilize metals as part of their bactericidal response, by sequestering essential iron and magnesium from bacteria and accumulating toxic levels of copper and zinc. However, M. tuberculosis blocks the normal phagosome maturation pathway, and instead survives and replicates inside macrophages. Based on the observation that copper resistance is required for virulence of M. tuberculosis in animal models, we hypothesized that M. tuberculosis has multiple copper resistance mechanisms. In order to better understand virulence of M. tuberculosis, we aimed to elaborate its copper homeostasis mechanisms. First, we characterized a multicopper oxidase and determined its role in copper resistance. Multicopper oxidases are involved in copper resistance and virulence in some pathogenic bacteria. The M. tuberculosis genome encodes one predicted multicopper oxidase, which is conserved among pathogenic mycobacteria. We showed that this protein indeed has multicopper oxidase activity and that it is required for copper resistance in M. tuberculosis. Further, the proteins required for copper entry into mycobacterial cells are unknown. We showed that general porins of M. smegmatis and M. tuberculosis are required for copper uptake. These results reveal important aspects of copper homeostasis mechanisms in M. tuberculosis.