Summary: The incorporation of selected particulates or charged polymers into qualitative and quantitative assay systems has been found to increase virus specific infectivity. The purpose of this study was to identify a mechanism(s) to explain the effect of charged particulates upon plaquing efficiency. The clay minerals kaolinite (K), montmorillonite (M), with the bacteriophage φX-174 and its host , were used as a model system. Enhanced bacteriophage infectivity (expressed as plaque-forming units) occurred at K and M concentrations in the agar overlay of 0·14-0·50 and 0·01-0·15 mg ml, respectively. A maximum increase of bacteriophage titre (to 180-230% of the control value) occurred at 0·18-0·20 and 0·06-0·08 mg mlK and M, respectively. Increased infectivity was related to the type of clay mineral and to the cation saturating the exchange complex in the order K homoionic (i.e. saturated with a single cation type) to Na> Mg> Kand M homoionic to Mg> Na> K. Enhanced infectivity of the bacteriophage was not related to the surface area of K nor to the external or total surface area of M. The increase of bacteriophage titre above that of the control was only minimally related to cation-exchange capacity, but was associated with the anion-exchange capacity of the clay. The increased plaquing efficiency is ascribed to the formation of high-density bacteria-clay microcosms which, in turn, modify bacteriophage contact and adsorption to the host cell.


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