Recruitment of both the ESCRT and autophagic machineries to ejecting Mycobacterium marinum.

Cytosolic Mycobacterium marinum are ejected from host cells such as macrophages or the amoeba Dictyostelium discoideum in a non-lytic fashion. As described previously, the autophagic machinery is recruited to ejecting bacteria and supports host cell integrity during egress. Here, we show that the ESCRT machinery is also recruited to ejecting bacteria, partially dependent on an intact autophagic pathway. As such, the AAA-ATPase Vps4 shows a distinct localization at the ejectosome structure in comparison to fluorescently tagged Vps32, Tsg101 and Alix. Along the bacterium engaged in ejection, ESCRT and the autophagic component Atg8 show partial colocalization. We hypothesize that both, the ESCRT and autophagic machinery localize to the bacterium as part of a membrane damage response, as well as part of a "frustrated autophagosome" that is unable to engulf the ejecting bacterium.

The autophagic machinery ensures nonlytic transmission of mycobacteria.

In contrast to mechanisms mediating uptake of intracellular bacterial pathogens, bacterial egress and cell-to-cell transmission are poorly understood. Previously, we showed that the transmission of pathogenic mycobacteria between phagocytic cells also depends on nonlytic ejection through an F-actin based structure, called the ejectosome. How the host cell maintains integrity of its plasma membrane during the ejection process was unknown. Here, we reveal an unexpected function for the autophagic machinery in nonlytic spreading of bacteria. We show that ejecting mycobacteria are escorted by a distinct polar autophagocytic vacuole. If autophagy is impaired, cell-to-cell transmission is inhibited, the host plasma membrane becomes compromised and the host cells die. These findings highlight a previously unidentified, highly ordered interaction between bacteria and the autophagic pathway and might represent the ancient way to ensure nonlytic egress of bacteria.

WASH-driven actin polymerization is required for efficient mycobacterial phagosome maturation arrest.

Pathogenic mycobacteria survive in phagocytic host cells primarily as a result of their ability to prevent fusion of their vacuole with lysosomes, thereby avoiding a bactericidal environment. The molecular mechanisms to establish and maintain this replication compartment are not well understood. By combining molecular and microscopical approaches we show here that after phagocytosis the actin nucleation-promoting factor WASH associates and generates F-actin on the mycobacterial vacuole. Disruption of WASH or depolymerization of F-actin leads to the accumulation of the proton-pumping V-ATPase around the mycobacterial vacuole, its acidification and reduces the viability of intracellular mycobacteria. This effect is observed for M. marinum in the model phagocyte Dictyostelium but also for M. marinum and M. tuberculosis in mammalian phagocytes. This demonstrates an evolutionarily conserved mechanism by which pathogenic mycobacteria subvert the actin-polymerization activity of WASH to prevent phagosome acidification and maturation, as a prerequisite to generate and maintain a replicative niche.