Rhinovirus plaque formation in WI-38 cells with methylcellulose overlay

A sensitive, reliable plaque assay system is described for five rhinoviruses using freshly prepared methylcellulose overlay and human embryonic diploid cells. Circular plaques with irregular edges, 2 mm in size, were formed by rhinoviruses 1A, 2, 6, and 13 after 6 or 7 days of incubation. A fifth rhinovirus, 17, formed a 1- to 2-mm feather plaque after 14 days of incubation. Plaque counts of rhinoviruses 1A and 13 were not affected by varying the pH of the overlay from 6.9 to 7.5. Plaque sizes and plaque-forming unit values of high passage rhinoviruses 1A and 13 were equivalent when tested at 26, 31, or 36 C. The rhinoviruses tested were sensitive to incubation at 40 C or heating at 50 C. Enhancement of plaques was observed when Mg(++) was incorporated into agar overlays, but enhancement did not occur when Mg(++) was added to methylcellulose overlays.