I BACTERIOPHAGES 



delicately balanced phase of viral growth in which the infected 

 host cell and its carried "prophage" multiply at the same rate has 

 been termed "lysogenesis" or "lysogeny." Infection leading to 

 lysogeny is now thought to produce a modification of the genetic 

 apparatus of the bacterial cell and often results in changes in 

 bacterial properties, a striking example being the conversion of 

 avirulent strains of diphtheria bacilli to toxigenic potency by in- 

 fection with an appropriate phage. 



Under certain conditions some bacteriophages can attack and 

 kill susceptible bacteria with no evidence of bacterial lysis or of 

 phage multiplication. In these circumstances the phage par- 

 ticle behaves as an antibiotic rather than as a virus. In fact 

 certain antibiotics produced by bacteria, the colicins and pyo- 

 cins, are rather similar to bacteriophages in certain of their 

 properties. For these reasons the colicins as well as phages will 

 be discussed in this book. 



2. Discovery of Bacteriophages 



There is no doubt that numerous early bacterio ogists saw 

 and described signs of phage action in bacterial cultures. How- 

 ever, no intensive investigation of these phenomena was under- 

 taken prior to the appearance of a brief but provocative paper by 

 F. W. Twort (1915). This British bacteriologist described an 

 acute infectious disease of staphylococci that produced marked 

 changes in colonial morphology. The infective agent was 

 filterable and could be passed indefinitely in series from colony 

 to colony. Twort considered various hypotheses to explain this 

 phenomenon; among others that it was a filterable virus analo- 

 gous to the virus pathogens of animals and plants. Twort's 

 remarkable paper contained in essence the present concept of the 

 nature of bacteriophage, yet the paper remained unnoticed by 

 scientists and Twort failed to pursue the matter further, perhaps 

 because of his wartime duties in the British Army. 



In 1917, Felix d'Herelle, a Canadian bacteriologist working at 

 the Pasteur Institute in Paris, published his independent dis- 

 covery of "bacteriophage." In a noteworthy series of papers he 



