242 BACTERIOPHAGES 



compensate for the bacterial deficiencies (Cohen, 1949; Gots 

 and Hunt, 1953; Burton, 1955). 



Although there is abundant evidence that an infecting bac- 

 terial virus can seriously alter the host cell metabolism, there is 

 still no clear cut evidence that the invading bacteriophage in- 

 troduces metabolically significant enyzme systems into the host 

 cells. Phages can also introduce bacterial genetic substances 

 controlling enzyme synthesis by means of the process known as 

 transduction, and prophages in lysogenic bacteria may have 

 profound effects on the metabolic activities of the host cell (Chap- 

 ter XIX). Indeed, the evidence furnished by work with tem- 

 perate phages makes it seem quite probable that enzymatic 

 activities uniquely associated with infection will be demon- 

 strated eventually. 



c. Enzymic Activity of the Host Cell 



It is now generally accepted that the energy requirement for 

 phage multiplication must be obtained through the functioning 

 of the enzymes of the host cell. Much of this evidence, obtained 

 by the use of enzyme inhibitors, will be described in Chapter XV. 

 However, one piece of evidence derived directly from an interest- 

 ing property of the phages is worth discussing here. The infec- 

 tion of a susceptible bacterium by certain virulent phages pre- 

 vents the synthesis of adaptive enzymes which are readily formed 

 in uninfected bacteria (Monod and Wollman, 1947). A strain of 

 E. coli infected with a virulent bacteriophage lysed and liberated 

 mature phage if glucose were present as an energy source. How- 

 ever, in the presence of lactose, phage growth and cell lysis 

 occurred only if the bacteria had been adapted to lactose utiliza- 

 tion before infection. Synthesis of the required enzyme took 

 place in uninfected cells within an hour after addition of lactose, 

 but did not occur at all in infected cells. In a further refinement 

 of this technique Benzer (1953) demonstrated that the rate 

 limiting factor in phage multiplication in a lactose medium is 

 the amount of induced enzyme synthesized before phage infec- 

 tion. These experiments show clearly that the lactose-hy- 



