The cell envelope of Mycobacterium tuberculosis with special reference to the capsule and the outer permeability barrier

This chapter concentrates on a few topics of current interest in the field of envelope studies, particularly the permeability of the cell envelope of Mycobacterium tuberculosis. A good deal of research was done on the plasma membranes of rapidly growing mycobacteria, but rather little is known specifically about the membrane of M. tuberculosis. There is every reason to suppose that mycobacterial membranes are structurally and functionally very similar to other bacterial plasma membranes. Mycobacterial peptidoglycan belongs to a family of structures possessed by almost all eubacteria but by no other type of living organism. The recent discovery of the mixture of proteins and polysaccharides that accumulates around unstirred M. tuberculosis in vitro, and presumably around the bacterium as it grows within a vacuole in a cell of its human host, is unexpected. Extraction of the envelope of M. tuberculosis with solvents releases a bewildering variety of substances, especially lipids and glycolipids. Porin-like proteins have been identified in the envelope of the rapidly growing nonpathogen M. smegmatis, and one of these has been sequenced. The investigation of mycobacterial porins, including those from M. tuberculosis, has already produced some exciting data, but our understanding is far from complete. M. tuberculosis, with its complex structure and metabolic capabilities and its ability to grow in several contrasting environments, is a fascinating subject for research.