FIG. 4.

Cytopathicity assay to assess the viability of L. pneumophila-infected human macrophages. U937 cell monolayers (n = 6) were infected at MOIs of 1 (A) and 10 (B) with wild-type strain 130b (●) and mutant NU260 (□). Macrophage viability is expressed as the percentage of alamarBlue dye reduction in infected compared to uninfected monolayers. Asterisks indicate significant differences in cytopathicity (P < 0.0005; Student's t test) between 130b and NU260. Comparable results were obtained from an additional experiment (data not shown). Error bars indicate standard deviations.