Removing ESX-5 alters tuberculosis membrane and boosts ESX-1 secretion

Tuberculosis remains a major global health challenge, and the bacterium that causes it, Mycobacterium tuberculosis (M. tb), has an unusual outer layer called the mycomembrane that helps it survive antibiotics and the immune system. Scientists are especially interested in a protein export machine called ESX-5, which is found only in slow-growing mycobacteria and is predicted to send more than 150 proteins across the inner membrane. To understand what ESX-5 does for the bacterium as a whole, a research team led by Benjamin Koleske used a complete locus deletion strain that removes ESX-5 from M. tb. By studying this M. tb Δesx5 mutant, the team could observe what happens when the entire ESX-5 secretion system and its many substrates are missing. The work focused on how loss of ESX-5 changes which proteins appear outside the cell and how the bacterial surface itself behaves. In this way the researchers sought to link molecular secretion changes to visible effects on the cell surface and to the bacterium's ability to keep out harmful compounds.

The core experimental approach reported in the abstract was the creation and analysis of a complete locus deletion strain, M. tb Δesx5. The researchers confirmed that PE/PPE proteins normally secreted by ESX-5 were selectively lost from both the culture filtrate (CF) and outer mycomembrane (OMM) fractions of the M. tb Δesx5 mutant. When they looked at other ESX systems, they found that ESX-1 substrate levels were increased in both the CF and OMM fractions of the Δesx5 mutant, indicating more ESX-1 cargo outside the cell. In contrast, the ESX-3 locus was transcriptionally repressed upon ESX-5 deletion, showing a reduction in ESX-3 expression. The M. tb Δesx5 mutant also displayed altered morphology, described as wrinkled distortions of the bacterial surface. Functionally, the Δesx5 mutant showed increased susceptibility to a variety of large (molecular weight >550 g/mol) antimicrobial compounds, suggesting an intact ESX-5 system is needed for M. tb to exclude such molecules from the cell.

Taken together, these observations indicate that removing ESX-5 fundamentally changes the properties of the mycobacterial outer mycomembrane and influences other secretion systems. Loss of ESX-5 not only prevents export of many PE/PPE substrates but also correlates with boosted ESX-1 substrate secretion and reduced ESX-3 expression, showing a cascade of effects across different ESX loci. The altered surface morphology and the increased vulnerability to large antimicrobial compounds point to a role for ESX-5 in maintaining outer membrane integrity and selective exclusion of certain molecules. Because the mycomembrane and secreted proteins are central to how M. tb interacts with drugs and the host immune system, the findings highlight ESX-5 as a central modulator of secretion and cell envelope composition. That central role makes ESX-5 a potential consideration for both drug targeting strategies and vaccine development, as changing ESX-5 activity reshapes the bacterial surface and secretion landscape.