Rhodococcus equi' is a Gram-positive soil bacterium that has the ability to infect foals by surviving in the alveolar macrophages, resulting in pyogranulomatous bronchopneumonia. Annotation of the 'R. equi' genome identified 2?3 two-component systems (TCSs) that are encoded on the chromosome. A TCS DNA microarray was developed to compare the gene expression profile of the bacteria in murine macrophage-like cells to growth ' in vitro'. This identified six sensor kinases as being significantly up-regulated during growth in the macrophage. The persistence in macrophages of the six mutants was compared to wild-type and one sensor kinases, MprB, was found to be required for intracellular survival and was confirmed by a mouse intravenous infection model. MprB likely regulates ' orf8' and 'vapA' expression from the virulence plasmid through its indirect effect on an alternate sigma factor promoter found on the virulence plasmid, Porf5. The results of this study also show that 'R. equi' responds to the macrophage environment differently from 'Mycobacterium tuberculosis'. MprB is the first sensor kinase identified as required for 'R. equi' virulence and intracellular survival.