Do test tube-grown pathogenic mycobacteria possess a protective capsule?

Intracellularly-growing pathogenic mycobacteria are surrounded inside host-phagosomes by a protective capsule, also called the ‘electron-transparent zone’ (ETZ). Whether this capsule is formed during active intracellular growth of these bacteria or is present even in test tube-growing bacilli is not yet elucidated. The present electron microscopic investigation used immunoelectron cytochemistry and also a new method using gelatin-Lowicryl embedding. This morphological study showed for the first time that test tube-grown pathogenic species (Mycobacterium avium and M. tuberculosis) are effectively surrounded by a capsule-like structure resembling intramacrophagic ETZ which was absent in the nonpathogenic species M. smegmatis and M. aurum.