390 S. GARD AND O. MAAL0E 



B. Salt Effects 



1. Stability as a Function of Salt Concentration and Ionic Composition 



The modifying effect of the ionic environment on different mactivation 

 processes has been stressed repeatedly. Outstandmg examples are furnished 

 by Adams (1949a), who showed that moderate changes in the ionic composi- 

 tion may reduce the heat inactivation rate one millionfold (see p. 385), and by 

 Jerne and Skovsted (1953), wlio discovered that the rate of inactivation by 

 antibodies is extremely sensitive to the ionic strength of the reaction mixture. 

 These examples alone suffice to demonstrate that the ionic composition of 

 the suspending medium is as important a factor to control as are temperature 

 and pH. 



Experiments by Burnet and McKie (1930), by Stanley (1935); by Gratia 

 (1940); by Lark and Adams (1953); by Friedman (1953); by Ruegamer (1954); 

 by Bachofer and Pottinger (1953, 1954a); and by Nortlirop (1954, 1955) concur 

 in showing that salts play a very significant role in determining the stability 

 and reactivity of viruses. In general, minimum stability is observed in the 

 absence of salts, or at certain relatively low concentrations of monovalent 

 cations (0-1 M or less); if this concentration is raised, or if small amounts of 

 divalent cations (10~* — 10~^ 31) are added, the stability is greatly increased. 

 Presumably, stabilization is due to the formation, on the virus surface, of 

 complexes between protein and cations. 



Inactivation due to unfavorable ionic environment can be reversible or 

 irreversible. In the case of reversibility, a clear distinction shoidd be made 

 between cases in which an inactivated state can be recognized, in which the 

 virus fails to initiate infection when mixed with susceptible cells under op- 

 timal conditions, and cases in which failure to initiate infection is due to 

 deficiencies in the test system. If the latter can be shown to be true, we are 

 probably dealing with a case of cofactor requirement, either for adsorption 

 of the virus to the sensitive cells or for an early step in the reproduction 

 process. As examples of this type of deficiency in the test system we may 

 mention the tryptophan requirement of phage T4 for adsorption onto E. coli 

 B cells (Anderson, 1945) and the Ca-ion requirement for the initiation of the 

 reproduction cycle in cells infected with phage T5 (Adams, 1949b). Systems 

 of this type are discussed in detail in Chapter II of Volume II. 



True reversible inactivation at low salt concentration has been described 

 by Puck (1949). Phage Tl, taken from a suspension m distilled water or in 

 dilute buffer, is irretrievably lost if mixed with susceptible cells under op- 

 timal conditions for adsorption and infection (in broth). The phage can be 

 shown to adsorb but no productive infection results. If the "inactive phage" 

 is incubated in broth, it recovers slowly but quantitatively; when sensitive 

 cells are added, normal productive infection results. Similarly, phage Tl, 



