CHAPTER I 



INTRODUCTION 



1. Definition 



The name bacteriophage was given by F. d'Herelle to a 

 bacteriolytic substance that he isolated from feces. Now usually 

 shortened to phage, it means "eater of bacteria" and refers to 

 the remarkable ability of bacteriophages to bring about lysis of 

 growing bacterial cultures. More remarkable still was 

 d'Herelle's evidence that the lysis is accompanied by produc- 

 tion of more phage ; the lytic agent is transmissible in series from 

 culture to culture of susceptible bacteria. 



Today phages are universally recognized to form a group of 

 bacteria-specific viruses, that is, ultramicrobes of diverse char- 

 acter exhibiting all the signs of a long history of manifold varia- 

 tion, adaptation, and specialization. D'Herelle himself sub- 

 scribed to this view only in part: he considered all bacterio- 

 phages to belong to a single variable species. In so doing he, 

 and many of the opponents of the virus theory, ignored or de- 

 nied the strongest evidence for it, with the unfortunate result 

 that some of the ostensibly fundamental research on phages was 

 for a decade or more directed along fruitless channels. 



The lytic cycle recognized by d'Herelle was such a striking- 

 aspect of bacteriophage activity that other aspects were largely 

 ignored. However, a nonlytic phase of reproduction has been 

 rediscovered recently, causing phage research to branch out in 

 new directions. Certain strains of bacteriophage when infect- 

 ing a susceptible bacterial culture are able to enter into an in- 

 timate symbiotic relationship in which the host cell continues to 

 multiply, carrying the virus intracellularly in a noninfective 

 condition for an indefinite number of cell divisions. This 



