THE INFECTIVE PROCESS 15 



plaque. If permitted to lyse before plating each bacterium 

 liberates a considerable number of phage particles each of which 

 will produce a plaque. Therefore the plaque count of succes- 

 sive samples remains constant until the end of the latent period 

 when lysis begins. From this time the plaque count increases 

 with each sample until all infected bacteria have lysed, after 

 which it remains constant at the new higher level. The final 

 count of liberated phage particles divided by the initial count 

 of infected bacteria gives the average burst size. 



The dilution of the adsorption mixture at the end of the ad- 

 sorption period is an essential feature of the experiment because 

 it prevents further adsorption. If the suspension of infected 

 bacteria were not diluted, much of the phage released by the 

 early lysing bacteria would be adsorbed onto yet unlysed bacteria 

 and initiate secondary cycles of infection. 



The antiphage antibody inactivates unadsorbed phage without 

 affecting the multiplication of adsorbed phage (Delbriick, 

 1945b). If unadsorbed phage were not inactivated the early 

 plaque counts would include unadsorbed phage as well as in- 

 fected bacteria and the estimate of the burst size would be too 

 small. 



The one-step growth technique has been discussed in some 

 detail because its significance has not been understood by all 

 phage workers. This technique enables one to determine very 

 simply the effect of changes in the physical and chemical en- 

 vironment on the duration of the infectious cycle and on the 

 yield of virus per infected host cell. 



2. Adsorption 



Adsorption of phage to host cell is the stage in the infectious 

 cycle which is most readily accessible to laboratory study and 

 hence best known. If adsorption is prevented the infectious 

 process cannot proceed and the host cells continue to multiply as 

 if the phage were not present. One crucial factor in adsorption 

 is the ionic environment, so one of the first things to consider in 



