Mycobacterium avium- and Mycobacterium tuberculosis-containing vacuoles are dynamic, fusion-competent vesicles that are accessible to glycosphingolipids from the host cell plasmalemma.

The vacuoles inhabited by viable Mycobacterium avium and Mycobacterium tuberculosis show limited fusion with endosomal and lysosomal compartments. This ability to regulate the maturation of their phagosomal compartments and restrict their differentiation into hydrolytically active vacuoles appears to correlate with the survival of the bacilli. Data presented in this current study demonstrate that despite the apparent isolation of mycobacterial vacuoles from the lysosomal network, they are dynamic, fusion-competent vesicles. Exploiting the ability of cholera toxin B subunit to bind to GM1 ganglioside on the macrophage plasmalemma, we demonstrate that these glycosphingolipids have ready access to the mycobacterial vacuoles. Entry into mycobacterial vacuoles is rapid, within 5 min of addition to the cells, and does not proceed through a brefeldin A-sensitive pathway. Furthermore, the gangliosides follow a route that differs from that taken by fluid-phase markers. TLC analysis gangliosides isolated from Mycobacterium-containing vacuoles, and IgG-bead phagosomes reveal similar profiles. These data indicate that rather than being fusion incompetent, mycobacterial vacuoles are actually highly dynamic, fusion-competent vesicles that behave like an extension of the recycling endosomal apparatus.