Mycobacterium tuberculosis, the causative agent of tuberculosis, can manipulate host cell death pathways as virulent strains inhibit apoptosis to protect its replication niche and induce necrosis as a mechanism of escape. In vitro studies revealed that similar to lytic viruses, M. tuberculosis has the ability to induce cytolysis in macrophages when it reaches an intracellular burden of ~25 bacilli. Base on this finding, we proposed the burst size hypothesis that states when M. tuberculosis invades a macrophage at a low multiplicity of infection it replicates to a burst size triggering necrosis to escape the cell and infect naïve nearby phagocytes, propagating the spread of infection. The first part of this study investigated if the in vitro observations of M. tuberculosis cytolysis were relevant to cell death of infected phagocytes during pulmonary tuberculosis in vivo. Mice infected with a low dose of M. tuberculosis revealed during TB disease, the major host cell shifted from one type of phagocyte to another. Enumeration of intracellular bacilli from infected lung cells revealed the predictions of the hypothesis were confirmed by the distribution of bacillary loads across the population of infected phagocytes. Heavily burdened cells appeared nonviable sharing distinctive features similar to infected macrophages from in vitro studies. Collectively, the data indicates that M. tuberculosis triggers necrosis in mononuclear cells when its number reaches the threshold burst size.

The previous study showed during the period of logarithmic bacterial expansion, neutrophils were the primary host cell for M. tuberculosis coinciding with the timeframe of the highest rate of burst size necrosis. The second part of this study examined this link by infecting mice with one of four different M. tuberculosis strains ranging in virulence. Mice infected with the most virulent strain had the highest bacterial burden and elicited the greatest number of infected neutrophils with the most extensive lung inflammation and greater accounts of cell death. Treating these mice with a bacteriostatic agent decreased the bacterial load and infected neutrophils in a dose-dependent manner indicating necrosis induced by virulent M. tuberculosis recruited neutrophils to the lungs. Infected neutrophils can serve as a biomarker in tuberculosis as evidenced by poorly controlled infection and increased severity of lung immune pathology.