APPENDIX 467 



The adsorption follows the equation until 90-99 per cent of 

 the virus at least has been adsorbed, provided the ratio of phage 

 to bacteria is not too high. This provision is necessary because 

 the ability of bacteria to adsorb phage is limited, the bacterium 

 becoming saturated when it has adsorbed between 50 and 200 

 phage particles. The equation is also limited apparently by 

 the fact that the phage is nonhomogenous with respect to rate of 

 adsorption, a small and somewhat variable portion being 

 relatively slowly adsorbed. Delbriick (1940a) investigated the 

 effect of the physiologic state of the host cell on rate of adsorp- 

 tion. The most rapid adsorption of phage to host cell seems 

 to occur when the host cell is in the logarithmic growth phase 

 and growing in a favorable medium. Adsorption is more rapid 

 in stirred adsorption mixtures than in mixtures at rest. How- 

 ever, T. F. Anderson (1949) has observed that violent agita- 

 tion, as in a Waring Blendor, will prevent adsorption of phage 

 T4 to the host cell. 



There are several independent methods for determining the 

 rate of adsorption of phage to bacterium which are applicable 

 under different conditions; these methods will now be de- 

 scribed in detail. 



7. Assay of Unadsorbed Phage 



This method of determining adsorption rate is rather direct 

 and is applicable under conditions in which 20-90 per cent of 

 the total phage present is adsorbed during the experimental 

 period. 



Procedure. The bacterial culture is temperature-equilibrated 

 and a sample taken for bacterial assay just before addition of 

 phage. An accurately known quantity of phage is added to the 

 bacteria so that the initial concentration of phage in the ad- 

 sorption mixture will be known. After thorough mixing, 

 samples of the adsorption mixture are removed at known time 

 intervals and diluted 1 :100 to stop the adsorption process. One 

 ml. amounts of the diluted samples are then centrifuged at 

 3,000 r.p.m. for 5 min. to sediment the bacteria and adsorbed 



