FIG. 1.

Cell-to-cell spreading efficacy of L. monocytogenes wild-type strain (top row) and plcBΔpro mutant (bottom row) in infected mouse fibroblast L2 cells in the presence of gentamicin. Infected cells were washed at 60 min postinfection and overlaid with a semisolid medium containing 2, 5, or 10 μg/ml gentamicin. At 4 days postinfection, the monolayer was stained with neutral red, a weak cationic dye that diffuses across intact membranes and cumulates intracellularly. Plaques are defined by the absence of staining, which is indicative of major cell damage or cell lysis.