Figure 1.

In vitro characterization of L. monocytogenes strains. (a) Growth of L. monocytogenes strains in BHI broth. The indicated strains were grown at 37 °C for 25 hours in BHI broth and the number of bacteria was determined at the indicated times. Data represent the mean ± SD colony forming units (CFU) per milliliter for three experiments performed in duplicate with similar results. (b–d) Intracellular infections in HeLa cells. (b) HeLa cells were infected with the indicated L. monocytogenes strains for 1 hour prior to intracellular bacteria being quantified by gentamicin protection assay at 2 hours post-infection. Data represent the mean ± SD CFU per well for one of three experiments performed in triplicate with similar results. *P < 0.05. (c) Plaque formation by L. monocytogenes strains. HeLa cells were infected with the indicated strains. After 1 hour, the infected monolayers were washed and overlaid with medium containing gentamicin and 0.7% agarose. At 72-hours post-infection, monolayers were stained with neutral red to allow for visualization of plaques. Wells were photographed and plaque sizes were measured at 96 hours post-infection. Numbers indicate the mean percentage plaque sizes ± SD normalized to 10403S from three independent experiments measuring >10 plaques/experiment. (d) Intracellular growth in HeLa cells. HeLa cells were infected with the indicated strains for 1 hour. Gentamicin was then added and at 2-hour time intervals post-infection, HeLa cells were lysed and intracellular bacteria were enumerated by plating dilutions of lysates. Data presented is the mean ± SD of three independent experiments performed in duplicate.