470 BACTERIOPHAGES 



before lysing bacteria can contribute to the plaque count. A 

 sample of phage stock should be diluted into the antiserum, 

 held at the same temperature for the same length of time, then 

 diluted and plated to insure that the antiserum is really in- 

 activating the required proportion of free phage. The number 

 of infected bacteria at each time interval is then subtracted 

 from the total phage input in the adsorption tube to give the 

 amount of unadsorbed phage remaining. The log per cent 

 unadsorbed phage is then plotted against time and the slope 

 used to calculate A" as above. 



This method is good for following the reaction from about 

 1 to 90 per cent adsorption of the phage, the principal limitation 

 being the ability of the antiserum to neutralize a high enough 

 proportion of the free phage in the time allowed. 



This method can be combined with method 1 to give directly 

 at the same moment the number of free phage particles and the 

 number of infected bacteria in the adsorption tube. Obviously 

 the sum of the free phage and the infected bacteria should equal 

 the total phage input if the procedure is valid. In fact, this 

 method may be used to demonstrate that antivirus serum has no 

 neutralizing effect on virus once it has become adsorbed to the 

 host cell. 



3. Determination of Proportion of Surviving Bacteria 



The distribution of phage particles among the various bacteria 

 in the adsorption mixture follows closely the Poisson di tribution 

 provided the multiplicity of infection is less than 2. Multi- 

 plicity of infection is defined as the average number n of phage 

 particles adsorbed/bacterium in the adsorption mixture. The 

 general Poisson formula is 



P{r) ="-^-" 

 r! 



in which P{r) = the proportion of bacteria adsorbing r phage 

 particles, and n = multiplicity of infection as defined above. 

 The only P{r) which can be determined experimentally is /*(0), 



