« BACTERIOPHAGES 



nently with definite genetic loci in the hereditary apparatus of the 

 host cell, and thus become part of that apparatus. Once 

 temperate phages have evolved they could be irreversibly 

 transformed into virulent phages by further mutations. It is 

 suspected that certain virulent phages have retained the ability 

 to undergo genetic recombination with host cells. Host range 

 mutations of a virulent phage might permit it to attack hosts 

 with which it had no genetic affinity whatever and so ultimately 

 to give rise to hypervirulent phages that have lost all resemblance 

 to any known bacterial strains. According to this hypothesis 

 different phage strains may have completely independent phylog- 

 enies and a given phage may be much more closely related to 

 its host cell phylogenetically than to other phages. It is clear 

 that if this hypothesis has any validity there may be no phylo- 

 genetic relationship between bacteriophages and any other type 

 of virus. The resemblances of phages to plant and animal 

 viruses may be superficial and due to their filling of similar 

 ecological niches. Referring to bacteriophages as bacterial 

 viruses may convey some information about their way of life 

 but it should carry no implications about their origins. 



5. Lysogeny 



From what has been said above it is clear that the phenomenon 

 of lysogeny has played a central role in the formulation of ideas 

 about bacteriophages. Because of its significance to all aspects 

 of phage research today, lysogeny must be discussed early in 

 this book. Literally, the term means production of lysis and 

 refers to the habit of bacterial strains that characteristically 

 produce a bacteriophage capable of lysing other bacterial 

 strains. Unfortunately the term has been applied to two en- 

 tirely distinct phenomena with consequent confusion in thinking 

 and definition. 



One of these phenomena results from the contamination of a 

 partially susceptible bacterial strain with a bacteriophage. In 

 such cases it is sometimes technically difficult to rid the bacteria 



