Retention of EsxA in the Capsule-Like Layer of Mycobacterium tuberculosis Is Associated with Cytotoxicity and Is Counteracted by Lung Surfactant

Mycobacterium tuberculosis, the pathogen that causes tuberculosis, primarily infects macrophages but withstands the host cell's bactericidal effects. EsxA, also called virulence factor 6-kDa early secretory antigenic target (ESAT-6), is involved in phagosomal rupture and cell death. We provide confocal and electron microscopy data showing that M. tuberculosis bacteria grown without detergent retain EsxA on their surface. Lung surfactant has detergent-like properties and effectively strips off this surface-associated EsxA, which advocates a novel mechanism of lung surfactant-mediated defense against pathogens. Upon challenge of human macrophages with these M. tuberculosis bacilli, the amount of surface-associated EsxA rapidly declines in a phagocytosis-independent manner. Furthermore, M. tuberculosis bacteria cultivated under exclusion of detergent exert potent cytotoxic activity associated with bacterial growth. Together, this study suggests that the surface retention of EsxA contributes to the cytotoxicity of M. tuberculosis and highlights how cultivation conditions affect the experimental outcome.