Fig. 4.

Cytopathogenicity of F. tularensis strains. Culture supernatants of J774 cells (A) or PECs (B) infected as described in Materials and Methods were assayed for LDH activity at 0, 24, and 48 h, and the activity was expressed as a percentage of the level for noninfected lysed cells (positive lysis control). At 0 h, infected as well as noninfected J774 cells showed ∼11% activity, which differed at most by 1% between strains (data not shown). For PECs, this activity was higher (∼39% activity of the infected as well as noninfected cells) but differed at most by ∼4% between strains (data not shown). Shown are means and standard deviations of triplicate wells from one representative experiment of two. The asterisks indicate that the cytopathogenicity levels were significantly higher than those of uninfected cells at a given time point, as determined by a 2-sided t test with equal variance (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001).