INACTIVATION OF VIRUSES 405 



the phenomenon for poliovirus vaccine production and testing are obvious. 

 There are some indirect consequences as well. In order to produce a safe 

 vaccine to be used in mass vaccinations, one has to extend inactivation into 

 a range where assay of survival ratios is no longer practicable. One has, there- 

 fore, to build the procedure on an extrapolation from the accessible part of 

 the inactivation curve. Obviously, it is of utmost importance that the latter is 

 determined with sufficient accuracy, and particularly that the potential 

 activity — the reproductive capacity — is recorded. When tissue cultures are 

 used for assay of remaining activity, it is of decisive importance that readings 

 are extended over sufficient periods of time. If this is not observed, titers will 

 come out too low and the rate of inactivation too high. This fact is the 

 most probable explanation of the discrepancies in the results reported by 

 different authors (Gard and Lycke, 1957; Gard, 1957). 



Another question of great theoretical and practical mterest is that of re- 

 activation. A priori, a reversible change in the configuration of the nucleic 

 acid does not appear to be particularly plausible. The possibility of reversion 

 or perhaps coimteraction of chemical alterations leading to a reduction in 

 avidity seems much more likely. Nagler (1957) reported that adjustment of 

 the inoculum to pH 9.3 reduced the incubation period of formaldehyde- 

 treated poliovirus to normal values without affecting the end-point titer. If, 

 in this case, activity were assayed, as is the custom in many laboratories, by 

 taking final tissue culture readings 6 or 7 days after inoculation, the pH ad- 

 justment would be considered to have brought about a reactivation. This 

 example illustrates both the need for a stricter definition of the term reactiva- 

 tion and the desirability in studies of this nature of the establishment of 

 optimal conditions for activity assays. Nagler's method of restoring the 

 avidity of the virus to normal values might be better described as "deatten- 

 uation" than as reactivation. A situation of this kind might be anticipated, 

 particularly with viruses dependent upon specific receptor mechanisms. 

 Further studies are obviously needed before any definite conclusions can be 

 drawn. 



Formaldehyde inactivation has been treated at some length, as this seemed 

 to be the best place to discuss the theoretical and practical problems encoun- 

 tered in the study of chemical inactivation of viruses. Speculation may seem 

 to have been carried farther than the comparatively meager facts permit. 

 The justification for this hes m the obvious potentialities of research in this 

 field to contribute to our knowledge of the nature of viruses. 



C. Protein-Denaturing Agents 



Several amides, aromatic acids, and detergents enter mto reversible com- 

 binations with proteins. Applied in low concentrations, they cause swelling 



