22 BACTERIOPHAGES 



multiplication of the variant cells. The mutant bacterial 

 population will be resistant to most of the phage particles, but it 

 not infrequently happens that a large phage population contains 

 a few variant phage particles with an extended host range, 

 capable of attacking the phage resistant bacteria. These 

 host range mutant phage particles will multiply at the expense of 

 the mutant bacterial population and may cause a second clearing 

 of the culture. The second lytic episode may be followed by a 

 third turbidity if the bacterium has produced a mutant which is 

 resistant to the host range mutant of the phage. Bacterial and 

 phage mutations will be discussed in more detail in later chapters. 

 The physiological state of the bacterial population is another 

 important factor in the lysis of fluid cultures. The latent period 

 is minimal and the burst size largest when the bacteria are grow- 

 ing in the logarithmic phase in a nutritionally adequate me- 

 dium. If the bacterial population has passed from the logarith- 

 mic growth phase into the stationary phase, the cells become 

 poor hosts for phage growth, and lysis becomes very slow or may 

 not occur (Delbriick, 1940b). 



7. Lysis on Solid Media 



If 10^ bacteria are spread on the surface of an agar plate, and 

 then incubated, they will grow as a multitude of minute colonies 

 which soon become confluent giving a uniform film or "lawn" of 

 bacterial growth. The bacteria will continue to grow until ex- 

 haustion of nutrients or accumulation of toxic products inter- 

 feres. If a single phage particle is placed on the plate early in 

 the growth of the bacterial film, it will adsorb to a bacterium 

 and initiate an infectious cycle. The phage progeny remain 

 localized at the site of lysis, and gradually radiate from this site 

 by a slow diffusion through the agar. In this process neighbor- 

 ing bacteria will be attacked and lysed, increasing the phage 

 population further. Eventually the zone of lysis may become 

 large enough in area to be readily visible to the naked eye as a 

 circular zone devoid of bacteria. These cleared areas were ob- 

 served by d'Herelle who called them "taches vierges" (virgin 



