APPENDIX 477 



fection by increasing the input ratio. At a multiplicity of 

 2.3, about 10 per cent of the bacteria are not infected, about 

 23 per cent are singly infected and the remainder are multiply 

 infected. At a multiplicity of 5, essentially all the bacteria are 

 infected and the proportion of singly infected bacteria is only 

 3 per cent. 



The adsorption period of 5 min. was selected only for con- 

 venience. The input ratio was selected to give the desired 

 multiplicity of infection on the basis of the known adsorption 

 rate of phage T2. With a slowly adsorbing phage such as T5 

 one would have to increase either input ratio or adsorption 

 time to reach the same multiplicity of infection. The adsorp- 

 tion time is limited by the necessity of completing the dilu- 

 tions before the end of the latent period. Also, a prolonged 

 adsorption time means that initiation of infection in the popu- 

 lation is spread over a considerable period and hence that the 

 rise period is also prolonged. If it is desired, for instance, to 

 infect at nearly the same time all the bacteria which are to be 

 infected, the adsorption period should be shortened to 1 min. 

 and the input ratio correspondingly greatly increased. 



Dilution of the adsorption mixture 1:10 into the serum tube 

 has 2 purposes. (/) The relative adsorption rate of the phage 

 is decreased to 1:10 because the bacterial concentration is 

 decreased by this factor, while the absolute adsorption rate is 

 reduced to 1 :100. (2) The antiserum present rapidly decreases 

 the amount of free phage remaining, thus effectively halting 

 adsorption. In the case of high input ratios and short adsorp- 

 tion periods where it is desired to halt adsorption sharply, 

 dilution should be 1 :100. 



Since the period of contact with the antiphage serum is de- 

 signed to inactivate essentially all of the free phage, the plating 

 of an appropriate dilution of this tube giv^es directly the number 

 of infected bacteria in the adsorption tube. If the total bacterial 

 concentration in the adsorption tube is known from accurate 

 assay just prior to infection, the proportion of the bacterial pop- 

 ulation which is infected is known and from this the multiplicity 



