Mycolic acids

Mycolic acids are long fatty acids found in the cell walls of the mycolata taxon, a group of bacteria that includes Mycobacterium tuberculosis, the causative agent of the disease tuberculosis. They form the major component of the cell wall of mycolata species. Despite their name, mycolic acids have no biological link to fungi; the name arises from the filamentous appearance their presence gives mycolata under high magnification. The presence of mycolic acids in the cell wall also gives mycolata a distinct gross morphological trait known as "cording." Mycolic acids were first isolated by Stodola et al. in 1938 from an extract of M. tuberculosis.

Mycolic acids are composed of a longer beta-hydroxy chain with a shorter alpha-alkyl side chain. Each molecule contains between 60 and 90 carbon atoms. The exact number of carbons varies by species and can be used as an identification aid. Most mycolic acids also contain various functional groups.

M. tuberculosis produces three main types of mycolic acids: alpha-, methoxy-, and keto-. Alpha-mycolic acids make up at least 70% of the mycolic acids of the organism and contain several cyclopropane rings. Methoxy-mycolic acids, which contain several methoxy groups, constitute between 10% and 15% of the mycolic acids in the organism. The remaining 10% to 15% of the mycolic acids are keto-mycolic acids, which contain several ketone groups.

Mycolic acids impart M. tuberculosis with unique properties that defy medical treatment. They make the organism more resistant to chemical damage and dehydration, and limit the effectiveness of hydrophilic antibiotics and biocides. Mycolic acids also allow the bacterium to grow inside macrophages, effectively hiding it from the host immune system. Mycolate biosynthesis is crucial for survival and pathogenesis of M. tuberculosis. The pathway and enzymes have been elucidated and reported in detail. Five distinct stages are involved. These were summarised as follows:

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