FIG. 7.

Growth of L. pneumophila strains within U937 cells. Macrophage monolayers were infected in triplicate with 130b (circles), NU216R (triangles), and NU244 (squares) from 3-day-old BCYE agar plates. Bacterial CFU per monolayer were quantified at 0, 24, 48, and 72 h after inoculation. Each data point represents the mean ± standard error for three independent monolayers. The 48-h time point for NU244 was identical to that for 130b in two independent experiments and as such does not represent a significant difference from 130b. The difference at t = 0 between 130b and NU216R is consistent with the observation that the wild-type inoculum contained proportionally more bacteria. Significant differences in recovery were observed between 130b- and NU216R-infected macrophages at all times except t = 0 h (P < 0.05; Student's t test).