The intracellular pathogen Rhodococcus equi increases phagosome membrane proton permeability

ABSTRACT Rhodococcus equi multiplies in macrophages of foals and human AIDS patients. Neutralization of the pH within R. equi -containing phagosomes is a prerequisite for intracellular multiplication. Previous studies have shown that R. equi factors permeabilize ex vivo model membranes for substances the size of hydrated calcium ions. Here, we show that live, but not dead, R. equi permeabilizes phagosome membranes during infection of macrophages, allowing the exchange of protons or acid equivalents between the phagosome lumen and cytosol. IMPORTANCE Phagocytic cells internalize intracellular pathogenic bacteria into a phagosome. Once taken up, the pathogens redirect phagosome development and establish a compartment of their own. Using modified fluorescence ratiometry, we show here that the foal lung pathogen Rhodococcus equi permeabilizes its surrounding phagosome host membrane for proton passage during infection of macrophages. This process requires bacterial viability. Membrane permeabilization explains how the phagosome lumen attains a neutral pH, which the bacteria require for multiplication.