FIG 6.
Comparative virulence of three passaged and three nonpassaged D39 cultures prepared from a single freezer stock. Mice were infected intranasally with 1 × 106 CFU in 50 μL of PBS. Lung (A to C) and blood (D to F) bacterial burdens were determined at 24 h postinfection. Data are from a single experiment using cultures prepared from agar plate 1 (A and D), agar plate 2 (B and E), or agar plate 3 (C and F), as depicted in Fig. 5. *, P < 0.05; **, P < 0.01; ns, nonsignificant (by an unpaired two-tailed t test).