Schematic model of the cell envelope of M. leprae. Schematic model of the cell envelope of M. leprae. The plasma membrane is covered by a cell wall core made of peptidoglycan covalently linked to the galactan by a linker unit of arabinogalactan. Three branched chains of arabinan are in turn linked to the galactan. Mycolic acids are linked to the termini of the arabinan chains to form the inner leaflet of a pseudolipid bilayer. An outer leaflet is formed by the mycolic acids of trehalose monomycolates (TMM) and mycocerosoic acids of phthiocerol dimycocerosates (PDIMs) and PGLs as shown. A capsule presumably composed largely of PGLs and other molecules such as PDIMs, phosphatidylinositol mannosides, and phospholipids surrounds the bacterium. Lipoglycans such as phosphatidylinositol mannosides, lipomannan (LM), and lipoarabinomannan (LAM), known to be anchored in the plasma membrane, are also found in the capsular layer as shown. (Reprinted from reference 425 with permission of the publisher.)‏ D. M. Scollard et al. Clin. Microbiol. Rev. 2006; doi:10.1128/CMR.19.2.338-381.2006