The cell envelope of tubercle bacilli

Summary The envelope of tubercle bacilli and of other mycobacteria is important for the bacterial physiology since inhibition of the production of some of its constituents kills the cells. It consists of a plasma membrane, which is apparently homologous to plasma membranes of other bacteria, surrounded by a complex wall of carbohydrate and lipid, which is in turn surrounded by an outermost layer, called ‘capsule’ in the case of pathogenic species. The wall possesses a fundamental, covalently linked ‘cell-wall skeleton’ composed of peptidoglycan covalently linked to arabinogalactan esterified by very long-chain (up to C90) fatty acids (mycolic acids). These fatty acids form the inner leaflet of a typical outer membrane (mycomembrane) whose outer leaflet consists of a great variety of non-covalently linked lipids and glycolipids. The thickness of the mycomembrane is similar to that of the plasma membrane, implying dedicated conformations of mycolic acids. Finally, a periplasmic space also exists in mycobacteria, between the membrane and the peptidoglycan.