FIG. 4.

Lysis of primary human neutrophils by ExoU⁺ and ExoU⁻ strains of P. aeruginosa. (A) Cell death was assessed by measuring the release of LDH. Each experiment was performed at least in triplicate. The background lysis in the absence of bacteria was subtracted from all values. The data are means ± standard errors of the means for four separate experiments. *, P < 0.01; †, P < 0.0001. (B and C) Cell death was assessed by microscopic examination of cells infected with PA99U in the presence of ethidium homodimer-1, which stains the nuclei red in cells that have lost membrane integrity, at 60 min (B) and 90 min (C) postinfection. Similar results were obtained in two separate experiments. Scale bars, 5 μm.