Mycobacterium tuberculosis, the causative agent of
tuberculosis, has two distinguishing characteristics: its ability to
stain acid-fast and its ability to cause long-term
latent infections in humans. Although this distinctive staining characteristic has often been attributed to its
lipid-rich cell wall, the specific
dye-retaining components were not known. Here we report that targeted deletion of kasB, one of two M.
tuberculosis genes encoding distinct beta-ketoacyl-
acyl carrier protein synthases involved in
mycolic acid synthesis, results in loss of
acid-fast staining. Biochemical and structural analyses revealed that the DeltakasB mutant strain synthesized mycolates with shorter chain lengths. An additional and unexpected outcome of kasB deletion was the loss of ketomycolic
acid trans-cyclopropanation and a drastic reduction in methoxymycolic
acid trans-cyclopropanation, activities usually associated with the trans-
cyclopropane synthase CmaA2. Although deletion of kasB also markedly altered the colony morphology and abolished classic
serpentine growth (cording), the most profound effect of kasB deletion was the ability of the mutant strain to persist in infected immunocompetent mice for up to 600 days without causing disease or mortality. This long-term persistence of DeltakasB represents a model for studying latent M.
tuberculosis infections and suggests that this attenuated strain may represent a valuable
vaccine candidate against
tuberculosis.