46 BACTERIOPHAGES 



7. Nonionizing Radiations 



Ultraviolet and visible light can also inactivate viruses. How- 

 ever, the mechanism of inactivation is quite different from that 

 of ionizing radiations. In the case of the relatively low energy 

 ultraviolet and visible radiations, entire quanta of radiation are 

 absorbed by specific chemical structures. Under suitable con- 

 ditions the energy absorbed may be enough to break chemical 

 bonds and inactivate viruses. Since the energy is absorbed only 

 by certain structures, the rate of killing gives no information 

 about the size of the virus particle. It was thought at one time 

 that there was an inverse relationship between sensitivity to 

 ultraviolet light and virus particle size. However, lambda, the 

 phage carried by the lysogenic strain K12 of £". coli, is 13 times 

 more resistant to ultraviolet radiation than is coliphage T5, 

 which is about the same size (Weigle and Delbriick, 1951). 

 The various physiological effects of ionizing and nonionizing 

 radiations will be considered in detail in Chapter VI. 



8. Weight of the Infectious Particle 



For those phages that have been isolated in highly purified 

 form, the dry weight of the infectious particle has usually been 

 determined. If the dry density is known the particle size can be 

 calculated from the relation 



Particle weight = 47rRV3 X density 



where R is the radius of a sphere of mass and density equal to 

 those of the dried phage particle. Sizes calculated in this way 

 from available data are summarized in Table III, and may be 

 compared with particle sizes of the same viruses obtained by 

 other methods and summarized in Table II. As might be 

 expected particle size estimated in this way is somewhat larger 

 than the size obtained by other methods. Any impurity in the 

 phage preparation would increase the dry weight per infective 

 particle, and any inactivation of phage during purification, or 

 an efficiency of plating less than unity, would have the same ef- 



