Seminar

Mycobacterium tuberculosis (Mtb), the leading cause of death globally by an infectious agent, evade host defense by hijacking and rupturing the engulfing phagosomal compartment of the host enabling it to escape. ESAT-6, a secreted virulence factor of Mtb, is known to be critical for phagosome rupture. However, the mechanism of disruption of the phagosome by Mtb remains unknown. Using in vitro reconstitution, live-cell imaging and numerical simulations, we discover that ESAT-6 undergoes polymerization and vesiculates phagosomal membrane. In sharp contrast to the canonical pore formation, we find that ESAT-6 binding induces phagosomal membrane shape transition leading to tubular and bud-like deformations facilitated by the reduction in membrane tension. Numerical simulations demonstrate that ESAT-6 polymerization generate both radial and tangential forces causing local remodeling and shape transition of the membrane. Furthermore, we quantitatively show that ESAT-6 binding induce local changes in membrane curvature and lipid phase separation that drive vesiculation. We also discover that the direct physical contact of the bacterial surface induces bending, lipid wrapping, and phase-separation at the host contact site. The degree of phase separation, driven by the bacterial load, scales with membrane fluidity and the degree of the phagosome deformation. Surprisingly, for saturating bacterial load the fluid host membrane transforms into a scaffold-like rigid membrane surface. Together, we propose that ESAT-6 polymerization forces and the bacterial surface induced phase separation drive the host phagosome rupture helping the bacteria escape.