The cell wall of M. tuberculosis consists of four layers—(i) pepti-doglycan layer, (ii) arabinogalactan layer, (iii) mycolic acid layer, and (iv) mycosides. The peptidoglycan layer is the innermost layer covalently linked with arabinogalactan (polysaccharide) and its terminal ends are linked to mycolic acid. This layer is overlaid with polypeptides and a layer of mycolic acid consist-ing of free lipids, glycolipids, and peptidoglycolipids. These lip-ids constitute nearly 60% of the dry weight of the cell wall.
The peptide chains present in the outer layer are impor-tant antigens, which stimulate cell-mediated immunity (CMI) in infected humans. PPDs are extracted and partially purified preparation of these proteins, which are used as antigens in tuberculosis skin test. The mycolic acid fraction of the lip-ids of the cell wall is responsible for many of the character-istic properties of the bacilli. These include (a) acid fastness, (b) slow growth by delaying permeation of nutrients, (c) resis-tance to commonly used antibiotics, (d) resistance to deter-gents, (e) unusual resistance to killing by acids and alkalies, and (f) clumping or cord formation.
M. tuberculosis strains are antigenically homogenous. They areantigenically similar to M. bovis and M. microti but are distinct from other species. Mycobacteria possess two types of anti-gens: (a) cell wall insoluble polysaccharide antigens and (b) cytoplasmic soluble protein antigens. Group specificity is due to polysaccharides present in the cell wall. These include ara-binogalactans, lipoarabinomannan, and also lipids, glycolip-ids, and peptidoglycolipids. Type specificity is conferred by