Growth of H7N7/ 03 viruses in cell culture. A) Plaque formation. MDCK, VERO or QM5 cells were infected with ~50 PFU of virus and stained with amido black after 7 days of incubation. B) Quantification of plaque size in MDCK, VERO and QM5 cells. The mean plaque size (measured in mm2) of chH7N7 is plotted relative to that of maH7N7 (set at 100%). Error bars represent the standard deviations. The asterisk indicates significant difference in mean plaque size (P < 0.05). C, D) Growth kinetics of H7N7/03 viruses. MDCK (C) or QM5 cells (D) were infected with chH7N7 or maH7N7 virus at an m.o.i. of 0.001 TCID50 per cell. Culture supernatants were harvested at 8, 24, 32, 48 and 72 h p.i. and virus titers were determined by end-point titration in MDCK cells. Shown are means from duplicate experiments with error bars indicating standard deviations. The dotted line indicates the lower limit of virus detection. Undetectable titers were assigned a value of 1.3.