INACTIVATION OF VIRUSES 379 



TMV was studied by Kleczkowski (1954). The rate constant for heat inactiva- 

 tion was found to increase with preirradiation, but the process was first order 

 also for suboptimal doses of UV. As pointed out by Kleczkowski, this sug- 

 gests that the "sensitization to heat inactivation" is a multi-hit pheno- 

 menon. Similar sensitization was observed with a Rhizobium phage (Klecz- 

 kowski and Kleczkowski, 1953) (compare discussion of the indirect effect of 

 X-rays, p. 367-69). 



The rate of adsorption and the bacteria-killing ability of phage T6 remain 

 unchanged after extensive UV inactivation (Fredericq, 1952); the enzymatic 

 activity of certain phages on the Vi substance is also very resistant to UV 

 (Kozinski and Opara, 1955). It is therefore reasonable to assume that these 

 phages are inactivated through absorption of UV in their nucleic acid moiety 

 before the surface properties are significantly damaged. 



6. Near UV and Visible Light (Wavelength above 3000 A). In a system 

 composed of virus particles and a suitable dye, light which is absorbed by the 

 dye may inactivate the virus. This photodynamic effect is quite different from 

 inactivation by direct absorption by the virus of UV, and is in some respects 

 similar to inactivation by radicals formed by ionizing radiation. 



Schultz and Kreuger (1928), Clifton and Lawler (1930) found that methylene 

 and toluidine blue — but none of a long series of other dyes — might be toxic 

 to phage particles. Further studies by Clifton (1931) revealed that oxygen is 

 essential for this type of inactivation and that cystine, in low concentration, 

 protects the virus. Divalent cations, which have a general stabilizing effect 

 on many phages, also increase their resistance to the photodynamic effect 

 (Burnet and McKie, 1930). Clifton concluded that the phage was inactivated 

 by oxidation caused by photo-sensitized dye in the presence of oxygen. This 

 theory has been further elaborated but not changed by later observations. 



Photodynamic inactivation of a variety of animal viruses, as mediated by 

 methylene blue, was studied by Perdrau and Todd (1933). The viruses of 

 vaccinia, herpes, fowl plague, loupmg-ill, Borna disease, Fujinami's tumor, 

 and canine distemper were fomid to be very sensitive, those of foot-and- 

 mouth disease and ectromelia much less. In susjDcnsions containing living cells 

 some viruses were protected, others not. 



The photodynamic inactivation of TMV in the presence of acriflavine was 

 studied quantitatively by Oster and McLaren (1950). At low salt concentra- 

 tion, about 160 dye molecules were foimd to absorb to each TMV particle 

 and exponential inactivation was observed uj^on illumination. At higher salt 

 concentration (> 0-1 M NaCl) the virus-dye complexes were partly disso- 

 ciated, with the result that the rate of inactivation dropped, although the 

 inactivating blue light was absorbed to the same extent. Oxygen was found 

 to enhance the effect greatly but some inactivation occurred even in a Ng 

 atmosphere. 



