Susceptibility of intracellular F. novicida to AR-12 varies at different time points post-infection. THP-1 macrophages were infected with F. novicida (dark bar) or a quadruplicate acid phosphatase deletion mutant (ΔABCH; gray bar) and treated with various doses of AR-12 for 3 h beginning at different time points post-infection. The number of surviving intracellular bacteria was determined by the CFU assay. Columns, mean; bars, ± SD (n = 3). *P < 0.05, **P < 0.01.