Effective generation of reactive oxygen species in the mycobacterial phagosome requires K+ efflux from the bacterium

Efficient killing of mycobacteria by host macrophages depends on a number of mechanisms including production of reactive oxygen species (ROS) by the phagosomal NADPH oxidase, NOX2. Survival of pathogenic mycobacteria in the phagosome relies on the ability to control maturation of the phagosome such that it is biologically and chemically altered in comparison to phagosomes containing non-pathogenic bacteria. In this study we show that the action of NOX2 to produce ROS in the mycobacterial phagosome is paradoxically dependent on a bacterial potassium transporter. We show that a Mycobacterium bovis BCG mutant (BCGDeltakef), deficient in a Kef-type K+ transporter, exhibits an increased intracellular survival phenotype in resting and activated macrophages, yet retains the ability to inhibit phagosome acidification, and does not show increased resistance to acidic conditions or ROS. Addition of a ROS scavenger replicates this phenotype in macrophages infected with wild-type BCG, and the production of ROS by macrophages infected with BCGDeltakef is substantially decreased compared with those infected with wild-type BCG. Our results suggest that increased intracellular survival of BCGDeltakef is mediated by inducing a decreased macrophage oxidative burst, and are consistent with Kef acting to alter the ionic contents of the phagosome and promoting NOX2 production of ROS.