[Comparative study of plaque formation by freshly isolated and laboratory strains of rubella virus].
The plaque-forming properties of 8 freshly isolated, 2 attenuated strains, RA 27/3 and Cendehill, and Sp-2 strain adapted to BHK-21/13C cell culture were studied. The concentration of sodium bicarbonate in the agar overlay influenced significantly the plaque production by rubella virus. Its optimum concentration was 0.22%. At higher (0.45%) and very low (0.11%) concentrations of sodium bicarbonate all the strains produced a smaller number of plaques. The strains were found to be sensitive to inhibitors in agar, the sensitivity being manifested in a smaller size of the plaques and reduction in their number. The strains were found to differ in the size and pattern of the plaques produced. Both among fresh isolates and attenuated strains, large-plaque (S+), small-plaque (S-) and S+/- strains were found. The S marker in combination with other markers may be used for differentiation of rubella virus strains.