Evidence that the capsule around mycobacteria grown in axenic media contains mycobacterial antigens: implications at the level of cell envelope architecture

The intracellular growth of pathogenic mycobacteria has been linked to the presence of an electron transparent zone (ETZ or capsule), which surrounds the phagocytized bacteria and prevents the diffusion of lysosomal enzymes in infected macrophages. Recently, it was suggested that this capsule may be a bacterial structures, even being present in test tube-grown pathogenic mycobacteria (FEMS Microbiol. Lett. 1988, 56, 225-230). In the present paper, we show that under special fixation and embedding conditions, this capsule was clearly observed among 7 strains of mycobacteria grown in axenic media and also in M. leprae extracted and purified from experimentally infected armadillo or nude mice. In the case of bacteria treated likewise but subject to a prior dehydration step, this capsular structure disappeared suggesting its lipidic nature. Ultrathin sections of M. intracellular after immunolabelling showed for the first time that this capsule obtained mycobacterial antigens confirming its mycobacterial origin. It is suggested that the mycobacterial capsule may be formed of inert lipids, in which surface antigens are embedded.