INACTIVATION OF VIRUSES 385 



stressed that the resistance observed in an optimally balanced salt solution 

 was at least as high as ui media to which has been added broth or similar 

 "protecting" substances. 



A particularly clear case of salt effect was later studied by Adams (1949a), 

 who found that the coliphage T5 is inactivated quite rapidly even at 37°C. 

 in a medium containing 0-1 M NaCl and less than 10~^ M CaCl_. Raising the 

 calcium ion concentrations to 10~^ M reduced the rate of thermal inactivation 

 by a factor of a miUion ! The same effect could be produced with an even 

 sharper transition by raising the NaCl concentration from 0-1 M to molar. 

 Adams concluded that the phage was stabilized by complexing with the 

 metal ions (see Section III, D, 2). Adams and Lark (1950) have shown that 

 T5 mutants exist which do not require this kind of stabilization; such mutant 

 phage stocks are as resistant m the absence as in the presence of calcium ions. 

 This is probably the only case in which genetic heterogeneity in a virus 

 population with respect to temperature sensitivity has been unequivocally 

 demonstrated. 



From inactivation rates estimated in the temperature mterval between 51 

 and 62°C., Krueger (1932) estimated the activation energy, AH, for staphy- 

 lococcus phages to be about 100,000 cal./mole, i.e., withm the range com- 

 monly found for protein denaturation. Similar and higher values were pub- 

 lished later, by Price (1940) for four plant viruses, by Bourdillon (1944) for 

 poHovirus, by PoUard and Reaume (1951) for the cohphages Tl, T2, T3, T4, 

 and T5, and by Welsch and Minon (1955a) for actino phages. 



In some cases it may be found that a single value for the heat of activation 

 does not suffice to describe the experimental data. For TMV, Price (1940) 

 found 55,000 cal./mole in the interval between 68 and 83°C. and 195,000 

 cal./mole between 84 and 95°C.; this situation, it has turned out, is not un- 

 common. It is usually mterpreted to mean that different inactivation pro- 

 cesses are rate-limiting in the different temperature intervals. Suggestive data 

 of this type have been obtained by Cherry and Watson (1949) for a strepto- 

 coccus phage, by Chang et at. (1950) for a coliphage, and by Bachrach et al. 

 (1957) for foot-and-mouth disease virus. For tobacco rmgspot virus. Price 

 (1940) also found two values but, m this case AH was significantly smaller m 

 the high than in the low temperature range; a simple interpretation in terms 

 of two independent inactivation processes does not suffice to explam this 

 situation. Even more complex are the data obtained by Kaplan (1958) for 

 vaccinia virus; in this case, the change m rate of inactivation in the interval 

 between 50 and 60°C. suggested a gradual decrease in activation energy. 

 Finally, it should be noted that single, well-defined, but remarkably low AH 

 values have been found for tobacco necrosis virus (37,000 cal./mole; Price, 

 1940), for influenza virus (34,000 cal./mole; Lauffer et al., 1948), and for 

 Theiler's virus strain FA (34,500 cal./mole; Leyon, 1951). 



