Plaque Enhancement of Enteroviruses by Magnesium Chloride, Cysteine, and Pancreatin

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Abstract
Plaque formation of 21 echoviruses (types 1-6, 9, 13, 15-19, 23-26, 29-32) and 8 coxsackieviruses (B1-6, A7, and A9) was enhanced by increased concentrations of MgCl2, L-cysteine, and pancreatin in agar overlay medium. In most cases, cationic and anionic polymers (diethyl-aminoethyl dextran, dextran sulfate or protemine sulfate) were ineffective. All strains of poliovirus and group B coxsackieviruses were enhanced under agar by MgCl2. Five of the eight coxsackieviruses tested were also enhanced by cysteine or pancreatin. Certain enteroviruses, which have been difficult to assay by plaque method, can now be quantified effectively by incorporation of additives such as MgCl2, cysteine, or pancreatin into the overlay medium.