FIG. 5.
(A) The pneumolysin-deficient ΔplnA mutant but not wild-type S. pneumoniae D39 was able to induce caspase-3 activation. Primary rat neurons were incubated with either D39, the ΔplnA mutant, or the ΔplnA ΔspxB double mutant at 107 CFU/ml, or with medium alone, for the indicated time points, and caspase-3 levels were determined using the fluorogenic caspase activity assay. Each error bar indicates the standard deviation for one of three experiments performed in triplicate. *, P < 0.05 for comparison with the control (Ctrl) or D39 by Student's t test. (B) D39 (107 CFU/ml, 6 h) potently induced apoptotic morphology, e.g., shrinkage of cells and nuclei, blebbing of nuclear membranes, condensation and fragmentation of nuclei (light microscopy bar, 30 μm; electron microscopy bar, 1 μm) but without activation of caspase-3.