FIG 4.

Effect of bacterial and host cell contact on the ability of VirB/D4 T4SS to inhibit caspase-3/7 activation in S. maltophilia-infected epithelial cells. Following the addition of A549 cells (A) or primary lung epithelial cells (B) into the lower chamber of a transwell apparatus, parental strain K279a (WT), virB10 mutant NUS15 (virB10), or complemented mutant NUS15(pvirB10) (virB10/virB10+) was added to the upper chamber of the transwell apparatus. A filter within the apparatus prevented contact between the bacterial inoculum and the host cells. At 24 h postinoculation, the levels of caspase-3/7 cleavage were determined by CellEvent caspase-3/7 green detection reagent. Uninfected (UI) A549 cells added to the lower chamber but not exposed to any bacteria served as a negative control. Asterisks indicate significant differences in the activation induced by the strains compared to that in UI cells (**, P < 0.01). As in Fig. 3, the data are normalized to the value for cells infected with the WT, which was set at 100% activation. Data are presented as the means and standard deviations of results from three independent experiments (n = 3 each).