20 BACTERIOPHAGES 



irradiation inactivation curves become multiple hit, indicating 

 that phage multiplication has started. 



5. At about the mid-point of the latent period, conversion of 

 vegetative phage particles into mature phage begins and the 

 number of infective particles found per cell increases until host 

 cell lysis occurs. 



It should be noted that this picture applies specifically to T2 

 and related coliphages. The details of the reproductive cycle 

 may be quite different for other phages. Much additional in- 

 formation about the reproduction of bacteriophages has been 

 obtained from nutritional studies, tracer experiments, and genetic 

 analysis, which will be reported later in appropriate chapters. 



5. Lysis of Host Cell and Release of Phage Progeny 



Of the last stage in the infectious process, lysis of the host cell, 

 almost nothing is known. Lysis is not dependent on the ac- 

 cumulation of mature phage particles because lysis will occur 

 even when phage multiplication is interrupted by chemicals such 

 as cyanide (Cohen, 1949) or proflavine (Foster, 1948). No 

 general method of temporarily interrupting the lytic process is 

 known except chilling, which slows down all enzymatic reactions. 

 Many chemical agents will interfere with phage growth but the 

 application of such agents soon leads to irreversible changes and 

 failure of lysis. 



The latent period of growth of certain phages can be greatly 

 prolonged by the phenomenon of "lysis inhibition" (Doermann, 

 1948a) but the mechanism of this is not understood. Lysis may 

 be induced before the end of the latent period by "lysis from 

 without" by massive doses of phage (Delbriick, 1940b), by sonic 

 vibration (Anderson and Doermann, 1 952a), or in certain cases by 

 enzymes such as lysozyme (Lwoff, Siminovitch, and Kjeld- 

 gaard, 1950). Towards the end of the normal latent period the 

 infected bacteria become increasingly fragile and some, lysis will 

 result from laboratory manipulations such as pipetting (Levin- 

 thai and Visconti, 1953). This pre-burst fragility of the bac- 

 terial membranes has led to the erroneous conclusion from the 



