Exploring the role of the mycobacterial ESX-2 secretion system in physiology and host-pathogen interaction

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, remains the leading cause of death from a single infectious agent, with approximately 1.25 million deaths annually. Although about 25 % of the global population is infected, only 5–10 % develop active disease, while in 90-95 % of cases, latent tuberculosis infection (LTBI) occurs. Type VII ESX secretion systems (ESX-1 to ESX-5), are key determinants of Mtb virulence and host adaptation. However, the role of the ESX-2 remains poorly understood. This Ph.D. project aimed at characterizing the physiological role of ESX-2 and its involvement in host–pathogen interaction by using genetic, molecular, and cellular approaches. Selected esx-2 genes were found upregulated under nutrient starvation, and during reactivation from hypoxia-induced dormancy. Consistently, ESX-2 mutant strains showed a reduced survival under nutrient deprivation conditions and delayed growth recovery following reactivation from hypoxia-induced dormancy, in vitro. Furthermore, ESX-2 mutants displayed impaired growth both in ex vivo models and in a murine infection model of infection, thus indicating the contribution of ESX-2 to optimal Mtb replication into the host. However, the expression of different ESX-2 variants (harboring engineered versions of the MycP2 ESX-2-associated protease) in Mycobacterium smegmatis did not allow the elucidation of the molecular mechanisms responsible for the modulation of the ESX-2 secretory activity.