382 S. GARD AND O. MAAL0E 



Stent (1953) and Stalil (1956) have actually shown that mactivation of a 

 phage particle by P^" decay can destroy any one of a large number of genetic 

 loci without affecting others. These experiments and a number of interesting 

 applications of the P^^ technique to the study of phage reproduction will be 

 discussed in another chapter. 



III. Physicochemical Factors 



0. Maal0e 



The description of a virus is not very useful or complete unless its stability 

 under different conditions is included. In this section we describe the most 

 important physicochemical factors to be considered when handling a virus in 

 the laboratory; i.e., the resistance of the virus to high and low temperature, 

 to desiccation, and to changes in pH and, last but not least, the importance 

 of the ionic composition of storage medium or of the medium in which the 

 virus is made to react with other agents. 



A. Heat Inactivation 



Heat inactivation of viruses may be discussed from two main viewj)oints: 

 First, it is an effective method for sterihzation and, when a proper balance 

 can be struck between destruction of uifectivity and preservation of useful 

 quahties of the preparation, heat treatment is convenient and cheap. Second, 

 studies of the temperature sensitivity is a natural step in the analysis of the 

 physicochemical properties of the virus. As usual, any of a number of 

 properties of the virus can be chosen as index of inactivation. 



1. Sterilization 



Sterilization of virus-contammg material is a problem of growing concern: 

 in the dairy and fermentation industries it may be a problem to get rid of 

 phages, especially such as infect lactic acid bacteria; and, in vaccine produc- 

 tion and in the manufacture of dairy products and certain pharmaceutical 

 products, it is essential to safeguard against contamination with pathogenic 

 viruses. In all such cases heat inactivation may be tried, alone or combined 

 with other treatments. 



A large number of streptococcus phages have been studied with the idea 

 of establishing practically useful "thermal death points" (Whitehead and 

 Hunter, 1937). The results of such tests are often reported in such terms as: 

 at pH 7, the activity of most phages was destroyed in 15 minutes at 70°C.; 

 in one case, it was destroyed in 5 minutes at 65°C. (Nelson et at., 1939). Like 

 their host organisms, streptococcus phages differ widely in heat resistance, 

 and some members of this group of phages will resist 82°C. for at least 5 



