MYCOBACTERIUM-AVIUM-CONTAINING 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 compart ments. This ability to regulate the maturation of their phagosomal com partments and restrict their differentiation into hydrolytically activ e 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 chol era toxin B subunit to bind to GM1 ganglioside on the macrophage plasm alemma, we demonstrate that these glycosphingolipids have ready access to the mycobacterial vacuoles. Entry into mycobacterial vacuoles is r apid, within 5 min of addition to the cells, and does not proceed thro ugh a brefeldin A-sensitive pathway. Furthermore, the gangliosides fol low a route that differs from that taken by fluid-phase markers, TLC a nalysis of gangliosides isolated from Mycobacterium-containing vacuole s, and IgG-bead phagosomes reveal similar profiles. These data indicat e that rather than being fusion incompetent, mycobacterial vacuoles ar e actually highly dynamic, fusion-competent vesicles that behave like an extension of the recycling endosomal apparatus.