432 BACTERIOPHAGES 



tcriophages may contribute useful evidence about the taxonomy 

 of bacteria (Stocker, 1955). Such studies have lead Girard 

 (1943) to question the present taxonomic position of the plague 

 bacillus. 



5. Special Taxonomic Criteria Applicable to Temperate Bacterio- 

 phages 



Some phages are able to establish lysogeny in the bacteria 

 they infect and are said to be temperate on this account (Chapter 

 XIX). This property can of course be used as a taxonomic 

 character. Its use is limited, however, by the fact that for al- 

 most every temperate bacteriophage which has been studied to 

 some extent, mutants are known that have lost the ability to es- 

 tablish lysogeny. A virulent phage could thus be a virulent mu- 

 tant of a yet unknown temperate phage. One may even enter- 

 tain the view that all phages are or were originally temperate, 

 and that virulence represents a secondary development due to 

 mutation. Against such a generalization stands the fact, how- 

 ever, that some phages are able to kill the host cell by merely ad- 

 sorbing onto the bacterium, even when active multiplication of 

 the phage is impossible. Such phages of course could not lyso- 

 genize in any case unless they first lost such an ability to kill. 



Other properties which are observed only with temperate 

 phages could presumably be used as taxonomic criteria. 



One of these is the inducibility of some temperate phages. Bac- 

 teria lysogenic for such phages, upon receiving certain stimuli 

 (ultraviolet light, X-rays, carcinogenic substances, etc.), lyse 

 after a definite latent period, liberating phage. Bacteria lyso- 

 genic for other phages do not react in this manner. To date the 

 property of inducibility is not known to be altered by mutation, 

 and may very well be a good taxonomic character. 



Another property which can be studied in temperate phages is 

 the ability of a phage to modify genetic properties of the host 

 cells it infects. This phenomenon may occur with various fre- 

 quencies, mechanisms, and specificities (Chapters XIX and 

 XXI). The difficulty with such properties from a taxonomic 



