An Infectious Centres Technique for the Assay of Infectious Polyoma Virus DNA

Most preparations of infectious virus nucleic acids retain only about 0.1% or less of the infectivity of the original virus material. This is not necessarily the result of destruction of much of the intact nucleic acid during chemical procedures, but may be due to the insensitivity of the assay methods employed (Holland et al. 1961; Tovell & Colter, 1967). For the titration of infectious DNA obtained from polyoma virus, one of the methods in current use requires the direct plating of DNA in 0.55 m-NaCl on the monolayers of mouse embryo cells (Weil, 1961). This procedure has the disadvantage that the cell damage caused by the hypertonic NaCl renders accurate plaque counts difficult. A more recently described method (Warren & Thorne, 1968) avoids this complication by infecting monolayers in the presence of diethylaminoethyldextran (DEAE-dextran) in an isotonic medium. The infectious centres technique of Ellem & Colter (1960) also overcomes this difficulty but the application of this method to the assay of infective polyoma DNA was unsuccessful (Weil 1961). In this communication a method is described for the titration of polyoma DNA using an infectious centres technique, the sensitivity of which is enhanced by the addition of an appropriate dilution of dimethyl sulphoxide (DMSO).