Ultra-low dose of Mycobacterium tuberculosis aerosol creates partial infection in mice

A murine low dose (LD) aerosol model is commonly used to test tuberculosis vaccines. Doses of 50-400 CFU (24 h lung CFU) infect 100% of exposed mice. The LD model measures progression from infection to disease based on organ CFU at defined time points. To mimic natural exposure, we exposed mice to an ultra-low dose (ULD) aerosol. We estimated the presented dose by sampling the aerosol. Female C57BL/6 mice were exposed to Mycobacterium tuberculosis H37Rv aerosol at 1.0, 1.1, 1.6, 5.4, and 11 CFU presented dose, infecting 27%, 36%, 36%, 100%, and 95% of mice, respectively. These data are compatible with a stochastic infection event (Poisson distribution, weighted R2 = 0.97) or with a dose-response relationship (sigmoid distribution, weighted R2 = 0.97). Based on the later assumption, the ID50 was 1.6 CFU presented dose (95% confidence interval, 1.2-2.1). We compared organ CFU after ULD and LD aerosols (5.4 vs. 395 CFU presented dose). Lung burden was 30-fold lower in the ULD model at 4 weeks (3.4 vs. 4.8 logs, p < 0.001) and 18 weeks (≤3.6 vs. 5.0 logs, p = 0.01). Mice exposed to ULD aerosols as compared to LD aerosols had greater within-group CFU variability. Exposure to ULD aerosols leads to infection in a subset of mice, and to persistently low organ CFU. The ULD aerosol model may resemble human pulmonary tuberculosis more closely than the standard LD model, and may be used to identify host or bacterial factors that modulate the initial infection event.