INACTIVATION OF VIRUSES 389 



with different freeze-drying equipments can only be compared if the tem- 

 perature maintained during sublimation is known. This factor was carefully 

 studied by Greiff e^ al. (1954) who found that the infectivity of influenza virus 

 was best preserved if the subhmation temperature was maintained near or 

 below - 80°C. 



As regards the polioviruses, Faber et al. (1951) found that drying at room 

 temperature from suspensions containing mucoid material or stools resulted 

 in complete inactivation, and Pollard (1951) found that infectivity as well as 

 antigenicity was reduced by a factor 100 or 1000 mider various freeze-drying 

 conditions. Much more promising results were obtained by Kraft and Pollard 

 (1954), who reported about 10 % recovery after drying of polio virus when 

 peptone or sodium thioglycolate was added to the suspending medium before 

 freezing or to the reconstituting fluid. 



C. Virus Stability and pH 



It is common experience that pH, like ionic environment and temperature, 

 greatly influences the stability of viruses, as well as their interaction with 

 other agents. As a general rule viruses will remain stable for hours, at tem- 

 peratures below about 20°C., if the pH of the suspending medium is between 

 5 and 6 on the acid and 8-9 on the aUcaline side. However, there are great 

 variations among viruses in this respect; if storage for long periods is envisa- 

 ged, the optimal pH should be determined by "accelerated degradation 

 tests," i.e., by subjecting the virus, at the desired pH values, to a tempera- 

 ture that produces a convenient inactivation rate. 



The general resistance pattern just described was found to apply to the 

 coliphages Tl and T4 (PoUard and Keaume, 1951; Gonnert and Bock, 1955) 

 to influenza, herpes, and Theiler viruses (Gonnert and Bock, 1955), and to 

 four adenoviruses (Ginsberg, 1956). A detailed study of influenza viruses by 

 Miller (1944) gave a more differentiated picture, showing that stability de- 

 pended strongly on the buffer used and that, as usual, infectivity was lost 

 before the hemagglutinating activity. Quantitative studies of the stability 

 of the coliphages of the T-series as function of pH and ionic environments 

 were carried out by Ruegamer (1954). Very striking differences between 

 closely related strains were observed; thus, the phages T4 and T6 were 

 stable, but phage T2 was rapidly inactivated at 2°C. in 2 X 10~^ M buffer 

 at pH 3. 



The remarkable instability of foot-and-mouth disease virus at slightly acid 

 reactions (pH 6-5-6-0) should be mentioned separately. Randrup (1954) has 

 shown that inactivation is due to the virus particles splitting up into non- 

 infective but serologically unchanged fragments. It is not known whether 

 other viruses undergo a similar degradation at low" j)H. 



