Direct contact of the bacterial surface induces phase separation in the host phagosome membrane

Pathogenic bacteria evade host defense by rupturing the phagosomal compartment, enabling their escape into the host cytosol. The bacteria is known to establish direct physical contact with the host compartment prior to phagosome disruption. However, the impact of direct-physical contact on the phagosome remodeling and deformation remains elusive. To probe this, we first developed a method wherein we reconstitute a phagosome-like giant compartment encapsulating Mycobacterium smegmatis, a non-motile opportunistic bacterium. We discover that the direct contact between the bacteria and the encapsulating host membrane induces membrane bending, lipid wrapping, and local lipid phase separation at the contact site. The degree of phase separation is driven by the bacterial load leading to fluidization of the membrane, as evident from the decreased area stretch and bending modulus, making the host compartment more deformable. Surprisingly, for saturating bacterial load the fluid host membrane transforms into a scaffold-like rigid layer. We also find that the direct contact of the bacteria enhances the membranolytic potential of ESAT-6 thus contributing to its virulence. Together our findings provide mechanistic insights into the role of direct physical contact of the bacteria during phagosome disruption.