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A mechanism involved in the plaque enhancement effect of foot-and-mouth disease viruses (FMDV) by the addition of sodium thiosulfate (Hypo) in the agar overlay medium (AOM) previously reported was studied. It was experimentally proved that the diffusion of virus particles through agar overlay medium was enhanced when this salt was incorporated. Accordingly, the enlarged plaque formation was assumed to be caused by the enhanced diffusion of viral progenies produced in infectious centers during plaque assays.

The viral susceptibility of a cell line, named KSEK6, newly established from the kidney cortex of swine embryo was tested for the indication of CPE occurrences and also plaque formations. The multiplication of porcine adenoviruses was considerably high in the cells among the virus strains tested though plaques of these viruses were hardly visible under agar overlay medium. Two strains of swine enteroviruses, Aujeszky's disease virus and hemagglutinating encephalomyelitis virus also multiplied well in a similar order to those received in the other cells employed.

A plaque enhancement effect by the addition of sodium thiosulfate for foot-and-mouth disease viruses was demonstrated when this salt was incorporated in agar and in agarose overlay media. Most of the mechanism is obscure, however, as one of the effects is that sodium thiosulfate seems to interact in a reversible manner against the plaque inhibitor action of neutral red in cellular cytoplasm. A plaque inhibitor contained in agar could be removed in some degree by the addition of this salt.