Structure of the cell envelope of Mycobacterium tuberculosis.

The cell envelope that surrounds the tuberculous bacillus is postulated to be part of the defense mechanism allowing the etiologic agent of tuberculosis to resist killing by phagocytic cells. It is composed of a plasma membrane, a wall formed by peptidoglycan covalently attached to arabinogalactan, which in turn is covalently linked to mycolic acids, and an outer layer. The molecular composition and the volume of this layer clearly differenciate Mycobacterium tuberculosis from pathogenic mycobacterial species, (also called capsule in this instance), from non-pathogenic ones. Recent studies have demonstrated that the outermost capsular components of the tuberculous bacillus consist mainly of proteins and polysaccharides, and not of lipids as previously thought, putting to question some of our current ideas on the chemical nature of the M. tuberculosis cell envelope.