PHAGE TAXONOMY 427 



constant characteristic of a phage-host cell system when deter- 

 mined under standard environmental conditions. However, 

 the latent period may vary with different host strains, with the 

 nutritional environment, the temperature, and the presence of 

 metabolic poisons. When a number of related phage strains 

 are compared it is found that the minimum latent periods fall 

 within a limited range of times which is characteristic for the 

 group. Because the range of minimum latent periods for dif- 

 ferent phage groups may overlap, this is not by itself a good tax- 

 onomic criterion but in conjunction with other properties may be 

 of value in characterizing a phage species. It suffers from the 

 common fault of all quantitative criteria in not being by itself 

 definitive. 



e. Susceptibility to Inactivation 



The susceptibility of phages to inactivation by various agents 

 such as heat, urea, photodynamic action, ultraviolet and ioniz- 

 ing radiations, sonic vibration, surface inactivation, changes in 

 salt concentration, and pH all have possible applications to 

 taxonomic problems (Chapters V and VI). The difficulty with 

 inactivation as a taxonomic criterion is that one phage strain 

 differs from another quantitatively rather than qualitatively. 

 Where these criteria have been applied to a group of related 

 phages there is usually found to be considerable variation among 

 the members of the group, and if several different groups are 

 compared there is quite likely to be overlapping in the ranges 

 of variation in the groups. This means that no single type of 

 inactivation, such as temperature sensitivity, can be used as a pri- 

 mary taxonomic criterion, although it may be expected to cor- 

 relate with classification by the serological and morphological 

 criteria. However, it is quite possible that a given taxonomic 

 group of phages may have a unique pattern of sensitivities when 

 a number of types of inactivation are studied. It may be antici- 

 pated on the basis of available evidence that susceptibility of 

 phages to inactivation will not be a very useful taxonomic cri- 



