{Mycobacterium smegmatis)

Fig. 6.22 Structure of two mycolic acids.

groups such as cyclopropane rings or methoxy groups. The acids are often named after the type of bacteria in which they are found (e.g. nocardomy-colic acid from Nocardia spp.).

The extractable free lipids comprise a mixture of cord factors, mycosides, sulpholipids and wax D. Together they represent about 25-30% of the mycobacterial cell wall. Cord factors are esters of the disaccharide, trehalose, with two mycolic acids (Fig. 6.23). Cord factor is so called because it is found in the waxy capsular material of virulent strains of tubercle and related bacteria. The factor causes the bacteria to string together in a long chain or cord. The compound is highly toxic and is somehow intimately associated with the virulence of the organism. It interacts strongly with host cell membranes thus impairing their function. Cord factor probably acts as a hapten and binds to albumin in plasma, thus forming an antigen. Like extracts of mycobacterial cell walls, cord factor acts as an immunostimulant. Killed mycobacteria (or their cell walls) are suspended in Freund's adjuvant, which is commonly injected with antigens to increase the titre of antibody.

The sulpholipids (Fig. 6.24) consist of trehalose, which is sulphated at the 2-position and acylated at several positions on both sugar residues. Instead of mycolic acids, the sulpholipids have a mixture of palmitic acid and very long chain (31C-46C) fatty acids with up to 10 methyl branches - known as phthioceranic acids.

Mycosides (Fig. 6.24) are again characteristic of mycobacteria. The basic structure for mycosides A and B is a long-chain, highly branched, hydro-xylated hydrocarbon terminated in a phenol group. Acyl groups are esterified to hydroxy groups of the hydrocarbon chain. In contrast, mycoside C is a glycolipid peptide.

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