Figure 3.

Listeriolysin O is required for L. monocytogenes strain LS741 intracellular infection and virulence. (a) Intracellular growth in HeLa cells. HeLa cells were infected with the indicated strains for 1 hour. Gentamicin was then added and at 2-hour 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. (b) L. monocytogenes LS741 Δhly is significantly attenuated in mice. BALB/c mice were infected intravenously with L. monocytogenes LS741 or LS741 Δhly bacteria (1–2 × 10⁴ bacteria/animal). At 72 hours post-infection the liver, spleen, and brain of each mouse were collected and the bacterial burden determined. Horizontal lines indicate median values. The Mann-Whitney test was used to assess statistical significance between bracketed groups: *P < 0.05, **P < 0.01. The dotted line indicates a limit of detection of 5 CFU per organ.