Polycation Effect on Virulence of Nonpathogenic r⁺ Variant of the Encephalomyocarditis Virus

Abstract

Takemoto and Liebhaber described two variants (r and r⁺) of encephalomyocarditis (EMC) virus which differ in the size of plaques formed in cultures of L cells¹. Plaques of r⁺ are 1 mm or less in diameter; those of r, 8 mm or greater. The small size of the r⁺ plaques is thought to be caused by sulphated acid polysaccharides in the agar overlay which bind the virus electrostatically and prevent attachment to neighbouring susceptible cells². Revertants appear when r⁺ is grown under liquid medium. Approximately 0.5 per cent to 2 per cent of the plaque-forming particles (PFP) in our pools either resemble r variant or form plaques intermediate in size between r⁺ and r. Growth of this latter revertant (rⁱ) after plaque purification is not inhibited by sulphated acid polysaccharides. Thus, inhibitor-insensitive plaque-forming particles invariably contaminate r⁺ pools at low dilutions but are not present at high dilutions.