Antimycobacterial action of thiolactomycin: an inhibitor of fatty acid and mycolic acid synthesis

Thiolactomycin (TLM) possesses in vivo antimycobacterial activity against the saprophytic strain Mycobacterium smegmatis mc2155 and the virulent strain M. tuberculosis Erdman, resulting in complete inhibition of growth on solid media at 75 and 25 micrograms/ml, respectively. Use of an in vitro murine macrophage model also demonstrated the killing of viable intracellular M. tuberculosis in a dose-dependent manner. Through the use of in vivo [1,2-14C]acetate labeling of M. smegmatis, TLM was shown to inhibit the synthesis of both fatty acids and mycolic acids. However, synthesis of the shorter-chain alpha'-mycolates of M. smegmatis was not inhibited by TLM, whereas synthesis of the characteristic longer-chain alpha-mycolates and epoxymycolates was almost completely inhibited at 75 micrograms/ml. The use of M. smegmatis cell extracts demonstrated that TLM specifically inhibited the mycobacterial acyl carrier protein-dependent type II fatty acid synthase (FAS-II) but not the multifunctional type I fatty acid synthase (FAS-I). In addition, selective inhibition of long-chain mycolate synthesis by TLM was demonstrated in a dose-response manner in purified, cell wall-containing extracts of M. smegmatis cells. The in vivo and in vitro data and knowledge of the mechanism of TLM resistance in Escherichia coli suggest that two distinct TLM targets exist in mycobacteria, the beta-ketoacyl-acyl carrier protein synthases involved in FAS-II and the elongation steps leading to the synthesis of the alpha-mycolates and oxygenated mycolates. The efficacy of TLM against M. smegmatis and M. tuberculosis provides the prospects of identifying fatty acid and mycolic acid biosynthetic genes and revealing a novel range of chemotherapeutic agents directed against M. tuberculosis.