Comparative growth and selection of small plaque and large plaque encephalomyocarditis virus in the absence of inhibitors from agar.

During multicycle growth of clones of encephalomyocarditis (EMC) virus forming large plaques (EMC/r), small-plaque-forming virus (EMC/r +) appeared and was always selected by conditions in the growth medium to become the predominant plaque type. EMC/r arose but was never selected by conditions in the growth medium to predominance in virus pools initiated by EMC/r + virus clones. Comparison of virus growth showed that EMC/r + was produced and released from EMC/r + infected L cells earlier and in 3to 5-fold higher yields than was EMC/r from L cells infected with EMC/r. Thus, when separately infected cells are considered, selection strongly favoured EMC/r + progeny. In mixed infection of L cells with EMC/r + and EMC/r, it was shown that: (1) Selection favouring the EMC/r + virus was reduced as the exposed multiplicity/cell of EMC/r + virus increased from 0.01 to 5.0 p.f.u./cell. (2) At 1-0 p.f.u./cell EMC/r and 5.0 p.f.u./cell of EMC/r + less virus of both types was formed in 8 hr than in controls with each virus alone at the same multiplicity. (3) Eight hr after infection more EMC/r was produced and/or released at 0.01 or 0.1 p.f.u./cell EMC/r and 1.0 or 5.0 p.f.u./cell of EMC/r +, than in control cultures without EMC/r + virus. ' Pure' ( > 99 ~o) pools of the EMC/r virus strain containing 109 to 10 I° p.f.u. were produced consistently by increasing the EMC/r virus pool by growth in consecutive single cycle steps, thus eliminating the selective conditions of multicycle growth.