FIG 1.

Extracellular L. monocytogenes bacteria perforate host cells in an LLO-dependent manner. Wild-type (wt) and LLO-deficient (Δhly) L. monocytogenes bacteria at various MOIs were incubated with HepG2 cells for 30 min at 37°C. The cell-impermeant dye propidium iodide (PI, 100 μM) was added to the cell culture medium during the last 5 min of incubation. After fixation and DAPI labeling, phase-contrast (PC) and fluorescence images were acquired with a 20× objective. (A) Representative images. Scale bar is 40 μm. (B) The average fluorescence intensity (A.F.I.) of PI was automatically measured in the nuclear region by quantitative fluorescence microscopy using Metamorph software. Results are expressed relative to the results for HepG2 cells incubated without bacteria (−). Results are the means ± SEM of at least three independent experiments performed in duplicate (*, P < 0.05; **, P < 0.01).