PDIM and EsxA drive distinct stages of vacuole damage in mycobacteria

Pathogenic mycobacteria initially occupy a membrane-bound compartment, the Mycobacterium-containing vacuole, where the ESX-1 effector proteins EsxA and EsxB cause damage that can be repaired by the host. A new infection study in the amoeba Dictyostelium discoideum identifies a distinct role for the cell-envelope lipid phthiocerol dimycocerosate (PDIM): rather than initiating membrane lesions, PDIM appears to help damaged vacuoles progress beyond host repair and toward catastrophic rupture. Both systems were required for full intracellular virulence, but their effects occurred at different stages.

To distinguish these stages, the researchers combined bacterial mutants with host reporters for membrane injury, endosomal sorting complexes required for transport (ESCRT) recruitment, autophagy and the acquisition of cytosolic perilipin. Removing EsxA/EsxB strongly reduced the recruitment of ESCRT and autophagy reporters to the Mycobacterium-containing vacuole. These observations indicate that the ESX-1 effectors initiate repairable membrane lesions and make them visible to host membrane-repair pathways. The finding places EsxA activity upstream of the recruitment of at least two repair systems rather than showing that EsxA simply disrupts vacuole integrity.

PDIM-deficient bacteria behaved differently. They still recruited early damage reporters and ESCRT machinery, suggesting that PDIM loss did not prevent the initial formation of membrane damage. However, the mutants showed reduced autophagy-associated repair, failed to acquire cytosolic perilipin coating efficiently and remained largely confined within the vacuole. They also induced recruitment of lysenin, a reporter of sphingomyelin exposure, but progression from that lesion to autophagy engagement was strongly decreased. Together, the results support a two-step model in which EsxA starts the injury process and PDIM enables lesions to evade or overcome host repair, increasing the likelihood of rupture, cytosolic escape and dissemination.

The genetic experiments also separated membrane damage from the host response that normally contains it. Disrupting host autophagy restored cytosolic access and increased the intracellular growth of PDIM-deficient bacteria, indicating that these mutants remained viable but were held back by autophagy-dependent repair. The same sequence was observed in murine microglial BV-2 cells, where the requirement for EsxA and PDIM at successive stages was conserved beyond the amoeba model. Because the study used bacterial mutants, reporter systems and targeted disruption of a host repair pathway, it resolves a previously unclear division of labor: EsxA initiates vacuolar membrane injury, whereas PDIM promotes its escalation toward catastrophic rupture. The work does not directly demonstrate outcomes in human infection, but it identifies an experimentally supported mechanism by which mycobacterial proteins and lipids can promote escape from a repairable intracellular niche.

Subject of Research: Distinct roles of EsxA and PDIM in mycobacterial vacuole membrane damage and host repair

Article Title: PDIM drives the transition from repairable to catastrophic EsxA-mediated vacuole membrane damage during Mycobacterium marinum infection

Article References: Michard, C., Perret, A., Foulon, M., Koliwer-Brandl, H., Hilbi, H., & Soldati, T. (2026). PDIM drives the transition from repairable to catastrophic EsxA-mediated vacuole membrane damage during Mycobacterium marinum infection. PLOS Pathogens, 22(10), e1014721. https://doi.org/10.1371/journal.ppat.1014721

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014721

Keywords: Mycobacterium marinum, PDIM, EsxA, ESX-1, vacuole rupture, host membrane repair, autophagy, ESCRT, cytosolic escape, mycobacterial infection, drives, transition