The Balance of Apoptotic and Necrotic Cell Death in <em>Mycobacterium tuberculosis</em> Infected Macrophages Is Not Dependent on Bacterial Virulence

<div><h3>Background</h3><p>An important mechanism of <em>Mycobacterium tuberculosis</em> pathogenesis is the ability to control cell death pathways in infected macrophages: apoptotic cell death is bactericidal, whereas necrotic cell death may facilitate bacterial dissemination and transmission.</p> <h3>Methods</h3><p>We examine <em>M.tuberculosis</em> control of spontaneous and chemically induced macrophage cell death using automated confocal fluorescence microscopy, image analysis, flow cytometry, plate-reader based vitality assays, and <em>M.tuberculosis</em> strains including H37Rv, and isogenic virulent and avirulent strains of the Beijing lineage isolate GC1237.</p> <h3>Results</h3><p>We show that bacterial virulence influences the dynamics of caspase activation and the total level of cytotoxicity. We show that the powerful ability of <em>M.tuberculosis</em> to inhibit exogenously stimulated apoptosis is abrogated by loss of virulence. However, loss of virulence did not influence the balance of macrophage apoptosis and necrosis – both virulent and avirulent isogenic strains of GC1237 induced predominantly necrotic cell death compared to H37Rv which induced a higher relative level of apoptosis.</p> <h3>Conclusions</h3><p>This reveals that macrophage necrosis and apoptosis are independently regulated during <em>M. tuberculosis</em> infection of macrophages. Virulence affects the level of host cell death and ability to inhibit apoptosis but other strain-specific characteristics influence the ultimate mode of host cell death and alter the balance of apoptosis and necrosis.</p>