Mycolic acids, the major
lipid of the Mycobacterium tuberculosis cell wall, are modified by
cyclopropane rings, methyl branches, and oxygenation through the action of eight
S-adenosylmethionine (SAM)-dependent
mycolic acid methyltransferases (MAMTs), encoded at four genetic loci.
Mycolic acid modification has been shown to be important for M.
tuberculosis pathogenesis, in part through effects on the inflammatory activity of
trehalose dimycolate (
cord factor). Studies using the MAMT inhibitor dioctylamine have suggested that the MAMT
enzyme class is essential for M.
tuberculosis viability. However, it is unknown whether a
cyclopropane-deficient strain of M.
tuberculosis would be viable and what the effect of
cyclopropane deficiency on virulence would be. We addressed these questions by creating and characterizing M.
tuberculosis strains lacking all functional MAMTs. Our results show that M.
tuberculosis is viable either without cyclopropanation or without cyclopropanation and any oxygenated mycolates. Characterization of these strains revealed that MAMTs are required for
acid fastness and resistance to
detergent stress. Complete lack of cyclopropanation confers severe attenuation during the first week after
aerosol infection of the mouse, whereas complete loss of MAMTs confers attenuation in the second week of
infection. Characterization of immune responses to the
cyclopropane- and MAMT-deficient strains indicated that the net effect of mycolate cyclopropanation is to dampen host immunity. Taken together, our findings establish the immunomodulatory function of the
mycolic acid modification pathway in pathogenesis and buttress this
enzyme class as an attractive target for antimycobacterial
drug development.