The ins and outs of the Mycobacterium tuberculosis-containing vacuole

Abstract

The past few years have seen publication of reports from several groups documenting the escape of Mycobacterium tuberculosis (Mtb) from its intracellular vacuole to access the cytosol. The major questions addressed in these publications are the mechanism(s) underlying this process, the frequency of its occurrence and, most importantly, the biological significance of this phenomenon to bacterial survival, growth and virulence. I believe that the first two questions are moving towards resolution, but questions relating to biological context have yet to be answered fully. In this viewpoint article, I will try to convince the readers why escape from the vacuole in no way diminishes the significance of Mtb's intravacuolar survival mechanisms and why, as a lab, we continue to focus the majority of our efforts on the 'bug in the bag'.

Figure 1

Electron micrographs of murine bone marrow-derived macrophages infected with Mtb CDC1551 and maintained in culture for 2, 4, and 14 days respectively. In panels A & B the cells were incubated with 15 nm colloidol gold for 2 hrs followed by a 45 min chase prior to processing. The gold identifies the lysosomal compartments that fail to fuse with the Mtb-containing vacuoles. At 14 days post-infection, panel C, even though the cell is carrying an impressive bacterial load, in this infection model the bacteria appear almost exclusively intra-vacuolar. Panel D illustrates the hypothesis that escape from the vacuole and the induction of necrotic cell death could play a critical role in late stage disease that in the mouse leads to death, and in humans leads to transmission. In this model the percentage of cell-associated Mtb that are intravacuolar represents the majority of organisms and is relatively constant until individual granulomas progress to active disease. Interestingly, in experimental infections with fluorescent reporter Mtb strains in non-human primates we do observed extracellular bacteria in the caseum. These bacilli are fluorescent but occur as singletons suggesting they are viable but non-replicating (unpublished data, Russell and Flynn).