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:

  • (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 %) pools of the EMC/r virus strain containing 10 to 10 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.


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