FIG 3.

Replication fitness of persistent virus populations. DF-1 cell cultures were infected (0.005 PFU/cell) with supernatants from naive DF-1 cells infected with WT virus (red) or from a 180-day- old DF-1P culture (blue). Infected cell supernatants were collected at the indicated times p.i. and used for virus titration. Each determination was carried out in triplicate. Presented data correspond to the means ± standard deviations from three independent experiments. Brackets indicate pairwise data comparisons. *, P < 0.01; **, P < 0.001; determined by two-tailed unpaired Student's t test. ns, not significant.