Plaque formation assay using different type I strains. (A) Example of plaques formed by RH-ERP2009 and GT1 when grown on HFF monolayers in six-well plates. On the left is a low-magnification image of one entire well. Scale bar, 1 cm. The image on the right was obtained by microscopic examination and shows a single plaque formed by GT1. Scale bar, 0.2 mm. (B) Quantification of the area of plaques developed by RH-ERP2009 and other type I strains. Plaques developed by RH-ERP77, RH-ERP88, and RH-ERP2009 were significantly (P ≤ 0.005) larger than other type I strains including from RH-JSR. Values are means ± the SEM (n = three separate experiments, 50 plaques/experiment) for RH-ERP77, RH-ERP88, RH-ERP2009, GT1, and GT1-LP, and means ± the standard deviation (SD) for each of two experiments, for remaining isolates.