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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 3- to 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:
‘ Pure ’ ( > 99 %) pools of the EMC/r virus strain containing 109 to 1010 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.