INACTIVATION OF VIRUSES 383 



minutes (Maze, 1946). The rate of inactivation of streptococcus phages has 

 not been studied directly, but Nichols and Wolf (1945) report that most of the 

 26 strains tested at a concentration of 10'^ viable particles/ml. were not com- 

 pletely killed in 15 minutes at 70°C. 



Yakovlev (1939) draws attention to the fact that big volumes are harder to 

 sterilize than small. (This results because, first, it takes time for a big volume 

 to reach the desired temperature; and second, the more particles originally 

 present (i.e., the bigger the volume), the more survivors will be found after a 

 given time of heating. This is common sense, and is mentioned only to em- 

 phasize how inadequate are statements in terms of thermal deathpoints. In 

 this connection it should also be recalled that, although inactivation usually 

 proceeds exponentially until few infective particles are left, these few are 

 frequently much more resistant than the majority (Kaplan, 1958). 



According to Prouty (1948), the heat resistance of most streptococcus 

 phages is such that normal commercial pasteurization is inadequate to 

 prevent phages entering dairy manufacturing establishments with contamin- 

 ated farmers' milk. A recent survey of the situation from this point of view 

 was published by Wilkowski et al. (1954). 



In certain media sterilization may be particularly difficult, as shown by 

 Whitehead (1944), who found that, m cream with 50 % fat, heating to 100°C. 

 with live steam was necessary to inactivate streptococcus phages. Such 

 special conditions of protection may also be a medical concern; thus, Kaplan 

 and Melnick (1954) have shown that, in ice cream, polio virus is considerably 

 more resistant than in aqueous suspension. In their experiments, aU 3 polio 

 strains retained some infectivity for cotton rats after being heated to 80°C. 

 in cream or in ice cream. 



The heat resistance of polioviruses in tissue culture filtrate has been studied 

 by Lepine and Nantel (1951). Their suggestion of a simple relation between 

 temperature and the time of heating necessary to obtain sterility is, however, 

 based on very hazardous extrapolations. Also, dried preparations of polio- 

 viruses have been tested for their heat resistance. Kraft and Pollard (1954) 

 have demonstrated that, at temperatures between 40 and 60°C. the rate of 

 inactivation decreases considerably with time, which shows that, m the dry 

 state, some particles are highly resistant. It is not known whether the resist- 

 ant particles differ genetically from the more sensitive. 



In vaccine production, sterility must be obtained without destroying anti- 

 genicity and, since effective heat inactivation usually involves a certain 

 degree of protein denaturation combined treatments are commonly used. 

 Thus, Salk (1946a,b) has shown that low concentrations of formahn consider- 

 ably increase the heat resistance of the hemagglutinating activity of influenza 

 virus. The usefulness of such "fixation phenomena" wiU be discussed in 

 detail in Section IV (inactivation by chemical agents). 

 VOL. 1—26 



