(A) Outbred Hartley guinea pigs were vaccinated intradermally with saline, BCG, or ΔsecA2 (n = 5 animals per group) and challenged by aerosol 2 months later with 10–30 CFU of virulent M. tuberculosis H37Rv. Graphs show SEM of CFU of M. tuberculosis in lung, spleen, and mediastinal lymph nodes at 1 and 2 months after challenge for naive (white bars), BCG-vaccinated (gray bars), or ΔsecA2-vaccinated (black bars) animals (CFU counts obtained from a minimum of 3 and in most instances 5 animals; an exception was 2-month spleen CFU for BCG-vaccinated animals, in which counts were obtained from only 1 animal for technical reasons). (B) Quantitative scoring of histopathology graphed as the SEM of the rank scores for the groups of guinea pigs vaccinated and challenged as described in A. Images show representative low-power (×25) views of H&E-stained sections of mediastinal lymph nodes harvested from each group at 1 or 2 months after challenge. Scale bars: 1 mm. *P < 0.05, **P < 0.01, ^†P < 0.001 compared with unvaccinated group or between bracketed groups; 1-way ANOVA with Tukey’s post-hoc test.