Enzymes for biosynthesis de novo and elongation of fatty acids in mycobacteria grown in host cells: is Mycobacterium leprae competent in fatty acid biosynthesis?

Fatty acid synthetase activity in extracts of Mycobacterium leprae was equivalent to 1.7 pmol malonyl-CoA incorporated into fatty acid min-1 (mg protein)-1. This activity--if representative of living M. leprae organisms--is insufficient to enable them to synthesize their lipid requirements rapidly enough to support growth. The major activity for scavenging fatty acids in extracts of Mycobacterium microti and Mycobacterium avium, as well as in extracts of M. leprae, was acetyl-CoA-dependent fatty acyl-CoA 'elongase'. This activity was about four times higher in M. avium and M. microti grown in a medium which contained lipids, or when grown in mice, than in medium without added lipids. In contrast, the de novo fatty acid synthetase activity was repressed in M. avium and M. microti when grown in medium that contained lipids, or when grown in mice. These results are consistent with the hypothesis that mycobacteria grown in vivo preferentially scavenge lipids from the host cells, and suggest that a source of lipid should be included in media for attempted axenic isolation of M. leprae.