APPENDIX 471 



which is the proportion of bacteria which adsorb no phage 

 particles, since these bacteria are capable of forming colonies 

 when plated on agar. For P(0) the Poisson formula simplifies 

 to 



P{0) = e~" 



so that n, the average multiplicity of infection, can be calculated 

 directly from the proportion of noninfected bacteria in the 

 adsorption mixture. The number of phage particles adsorbed 

 is equal to n y. B, where B is the total number of bacteria in 

 the adsorption tube. The number of phage particles adsorbed 

 divided by the total phage input gives the proportion of phage 

 particles adsorbed from which the rate of adsorption can be 

 calculated. 



Example: A bacterial culture of about 5X10' cells/ml. is brought to 37° C. 

 and a sample taken for accurate assay. Coliphage T2 is added to a final con- 

 centration of 5 X 10" particles/ml. and well mixed. After 5 min. adsorption 

 a sample is taken and diluted 1 :100 in the same medium to stop further ad- 

 sorption. It is then further diluted so that a 0.1 ml. sample would be expected 

 to contain between 50 and 200 surviving bacteria, in this instance a total dilution 

 of 10~*. Several 0.1 ml. samples of the lO"'' dilution are then spread on the 

 surface of agar plates with a glass spreader. The agar plates should be just pre- 

 viously spread with sufficient anti-T2 serum so that infected bacteria plated with 

 an excess of uninfected bacteria do not form plaques. This is to prevent free 

 phage and phage liberated from lysing bacteria from attacking the noninfected 

 bacteria. The amount of antiserum to use per plate must be found by trial. 

 The plates spread with the diluted adsorption mixture and antiserum are then 

 incubated and the colonies counted. By this technique the adsorption can be 

 followed right up to the end of the latent period of phage growth. In this 

 experiment, average colony count on the plates was 225, corresponding to a 

 survival of 2.25 X 10'' bacteria/ml. in the adsorption tube after 5 min. of ad- 

 sorption. Input of bacteria was 5.0 X lO'^ and the proportion not infected, 

 P(0) = 2.25/5.0 = 0.45 = e~". The corresponding value for n, the multi- 

 plicity of infection, is 0.8. The number of phage particles adsorbed is then 

 5X107X0.8 = 4X lOVml. Phage input in the adsorption tube was 5 X 

 lOVml.; hence the per cent adsorption in 5 min. at 37° C. was 80. 



Tables of values of e~'^ for various values of a' are available in 

 handbooks of physics and chemistry, or the values can be cal- 

 culated by use of log tables and the value of e which is 2.718. . . . 



