Thermal Inactivation Studies with Different Strains of Poliovirus

Summary Variations in the rate of destruction of infectivity at 50°C were demonstrated for strains representing the three immunologic types of poliovirus as well as for strains of the same type. These strain differences were consistent upon serial passage of the different virus lines. In addition, variations in thermal resistance of individual infectious units making up a given virus population have been investigated and it has been found that progeny of different virus particles vary in their rates of thermal inactivation. Utilizing selection pressure in the form of propagation of survivors of partially inactivated virus pools, it has been possible to select 50°-resistant variants, the progeny of which retain this characteristic. Other than temperature stability, no differences in properties of the parent and resistant lines could be demonstrated. Plaque morphology, rate of adsorption to monkey kidney cells, rate of destruction of infectivity and antigenicity by 1:4000 formalin, and inactivation at 36.5°C were the same for parent and resistant lines studied. Virus lines readily distinguishable at 50°C were inactivated at identical rates at 36.5°C. Survivor selection at 36.5°C failed to yield variants resistant to this temperature, or to alter the characteristic inactivation rate at 50°C. These results suggest a different mechanism for inactivation at the two temperatures employed.