For standard bacteriolysis assays, unprimed iIFNAR−/− macrophages, Caco-2 (epithelial) cells, or BHK (fibroblast) cells were plated at 5 × 105 cells per well in a 24-well plate and then infected with L. monocytogenes strains carrying the plasmid bacteriolysis reporter (pBHE573) or chromosomal bacteriolysis reporter (pYL56) at an MOI of 10, 64, or 100, respectively. The higher MOIs used in the Caco-2 and BHK cells were used to gain comparable levels of luciferase production similar to those in macrophage experiments. At 1 h postinfection, cultures were treated with 50 µg/ml gentamicin. At 6 h postinfection (iIFNAR−/− macrophages and BHK cells) or 8 h postinfection (Caco-2 cells), cells were assayed for luciferase activity as previously described (8). If indicated, infected cells were treated with 1 mg/ml ampicillin at 2 h postinfection.