Figure 4

Thermal stability of the mutant viruses. A Nine 60-μL aliquots of mutant viruses were exposed to 56 °C for 150 min. All aliquots were tested by hemagglutination assay. The error bars denote SD of the mean of three independent titers at each time point. B, C The wild-type virus and mutant viruses were incubated at 37 °C (B) or 42 °C (C) for 5 days. The TCID₅₀ titers of the aliquots were determined in CEF cells. D, E The wild-type virus and mutant viruses were diluted to the same TCID₅₀ and inactivated by methanol. Then all viruses were exposed to 37 °C (D) or 42 °C (E) for 18 h and every aliquot was collected every 2 h. The titers of the aliquots were determined by hemagglutination assay. An asterisk indicates that the titer of the mutant virus was significantly different from those of the wild-type virus at the time points indicated, as determined by the Mann–Whitney test (*p < 0.05, **p < 0.01).