The ESX-4 substrates, EsxU and EsxT, modulate Mycobacterium abscessus fitness

ESX type VII secretion systems are complex secretion machineries spanning across the mycobacterial membrane and play an important role in pathogenicity, nutrient uptake and conjugation. We previously reported the role of ESX-4 in modulating Mycobacterium abscessus intracellular survival. The loss of EccB4 was associated with limited secretion of two effector proteins belonging to the WXG-100 family, EsxU and EsxT, and encoded by the esx-4 locus. This prompted us to investigate the function of M. abscessus EsxU and EsxT in vitro and in vivo. Herein, we show that EsxU and EsxT are substrates of ESX-4 and form a stable 1:1 heterodimer that permeabilizes artificial membranes. While expression of esxU and esxT was up-regulated in M. abscessus-infected macrophages, their absence in an esxUT deletion mutant prevented phagosomal membrane disruption while maintaining M. abscessus in an unacidified phagosome. Unexpectedly, the esxUT deletion was associated with a hyper-virulent phenotype, characterised by increased bacterial loads and mortality in mouse and zebrafish infection models. Collectively, these results demonstrate that the presence of EsxU and EsxT dampens survival and persistence of M. abscessus during infection. The evolution of bacterial secretion systems is strongly associated with the intracellular lifestyle and interactions of bacteria with their hosts. Here, we show that an ancestral type VII secretion system, ESX-4, modulates the pathophysiology of Mycobacterium abscessus infection, which is directly related to its major effectors, EsxU and EsxT. While EsxU/ EsxT were found to permeabilize lipid bilayers, deletion of the esxUT genes increased the virulence ofM. abscessus in animal models. In support of these observations, strictly pathogenic mycobacteria in humans and animals, such as Mycobacterium tuberculosis or Mycobacterium marinum have evolved without evidence of secretion of their respective EsxU/EsxT substrates. Together, these results underscore the possible in vivo advantage procured by the absence of EsxU and EsxT in strictly pathogenic mycobacterial species.

Automated summary

ESX type VII secretion systems play an important role in pathogenicity, nutrient uptake and conjugation in mycobacteria. EsxU and EsxT are substrates of ESX-4 and form a stable heterodimer that permeabilizes artificial membranes. Deletion of the esxUT genes increases the virulence of M. abscessus in animal models.