(A) C57BL/6 mice were vaccinated subcutaneously with saline (naive) or with 1 × 10⁶ BCG or ΔsecA2 and challenged by aerosol 2 months later with 50–100 CFU of virulent M. tuberculosis Beijing/W strain (HN878). Graphs show means and SDs of CFU of M. tuberculosis in lung and spleen at 1, 3, and 5 months after challenge for groups of 5 mice that were either naive (white bars), BCG vaccinated (gray bars), or ΔsecA2 vaccinated (black bars). *P < 0.05 and ^†P < 0.001, compared with unvaccinated group or between bracketed groups; 1-way ANOVA with Tukey’s post-hoc test. (B) Lungs of mice vaccinated and challenged with virulent M. tuberculosis as in A were examined histologically at 1 and 3 months after challenge. More severe, spreading lung lesions with extensive granulomatous pneumonia and consolidation were observed in unvaccinated mice as compared with mice vaccinated with either BCG or ΔsecA2. Original magnification, ×20. (C) C57BL/6 mice were vaccinated subcutaneously with saline or with 1 × 10⁶ BCG or ΔsecA2 and challenged by aerosol 2 months later with 50–100 CFU of virulent M. tuberculosis Erdman strain. Mice were observed daily for survival for 1 year after challenge. Moderate extension of survival was observed in BCG-vaccinated animals (P = 0.006 compared with naive control group; log rank test), and much more pronounced extension of survival was observed in ΔsecA2-vaccinated animals (P = 0.0001 compared with naive; P = 0.0062 compared with BCG).