Figure 4

In vitro assessment of the genetic stability of ribavirin-resistant PRRSV mutants during serial passages in cells. The genetic stability of the plaque-purified, ribavirin-resistant mutants (RVRp13-P and RVRp22-P) was compared with that of plaque-purified VR2332 (VR2332-p) over 10 passages in MARC-145 cells. Fifteen viral clones obtained from cell culture fluids collected at the completion of 10 passages of each virus by plaque purification and nsp2 (A) and ORF5 (B) regions of each virus clone were sequenced and compared with their parental viruses (RVRp13-P, RVRp22-P, and VR2332-p) to determine nucleotide mutations (a) and amino acid (b) substitutions during the sequential passages. Asterisks represent significant differences in the mutation rate compared with VR2332 (* p < 0.05, ** p < 0.001).