INACTIVATION OF VIRUSES 391 



inactivated in dilute buffer at pH 3-8, is gradually reactivated upon addition 

 of CaClg (Ruegamer, 1954). The experiments of Northrop (1954, 1955) showed 

 that several megatherium phages were reversibly inactivated at pH 5-8 in 

 media of low salt concentration. 



Irreversible inactivation at low or intermediate salt concentrations has 

 been observed several times. Gratia (1940) found several phage strains to be 

 rapidly inactivated at NaCl concentrations between 0,25 M and 0,03 M; very 

 low concentrations of Ca or Mg salts prevented inactivation. Friedman (1953) 

 made very similar observations with several megatherium phages. Lark and 

 Adams (1953) showed that phage T5, inactivated at low salt concentration, 

 has lost its ability to adsorb onto sensitive bacteria and that, in addition, 

 its DNA has been released into the medium. This double effect suggests that 

 inactivation may be due to damage to the tail tip by which the normal phage 

 adsorbs and through which the DNA of the damaged phage may be assumed 

 to leak out. 



Most studies on virus stability at low salt concentrations have been carried 

 out with phage. It should be mentioned, however, that a thorough investiga- 

 tion of the stability of centrifugally purified influenza virus in various buffers 

 of low ionic strength was made by Knight (1944). Reactivation was observed 

 upon addition of 0-1 M buffer to virus which had been partially inactivated 

 in distilled water. 



2. Osmotic Shock 



We have twice referred to coliphages as "losing their DNA," thereby being 

 transformed iato "ghosts" consisting of empty phage heads with tails still 

 attached. This splitting of phage particles into a protein membrane and tail 

 plus free DNA can be achieved by intense UV or sonic irradiation of the 

 T-even phages (Anderson, 1945; Anderson et al., 1948; see p. 363), by heat 

 treatment of phage T5 in the absence of Ca ions (Lark and Adams, 1953; see 

 preceding section); under certain conditions by freezing and thawing (Panijel 

 etal., 1957); and in some cases by osmotic shock. 



This phenomenon is observed if a suspension of one of the T-even phages 

 in a concentrated NaCl solution is diluted into distilled water (Anderson, 

 1949). The transition from high to low osmotic pressure must be rapid, hence 

 the term osmotic "shock." For the shock to be effective the osmotic pressure 

 must be reduced by about 70 atmospheres, or more, and the temperature at 

 which the phage is equilibrated in the concentrated solution must not be 

 above about 30°C.; NaCl may be replaced by any of a number of ionic and 

 nonionic solutes (Anderson et al., 1953). Under favorable conditions the 

 shock inactivates 98-99 % of the phage particles. 



The effect of a sudden drop in osmotic pressure may be explained by 

 assuming that the outer membrane of the phage particle is much more 



