APPENDIX 453 



A method for determining the relative efficiency of plating 

 when this is of the order of 0.1-1 and not due to lack of ad- 

 sorption cofactors or nutritional deficiencies was described by 

 Ellis and Delbruck (1939). In this method, the phage is so 

 diluted in broth that there is somewhat less than 1 infectious 

 particle/ml. and then distributed in tubes to give 50 samples of 

 1 ml. each. Each sample is inoculated with a few hundred 

 bacteria and incubated overnight. At the end of this time a 

 0.1 ml. sample from each tube is plated in the usual way to see 

 whether phage is present or not. The proportion of samples 

 containing no phage is determined and the average number of 

 phage particles per sample is calculated from the Poisson 

 formula 



P(0) 



in which P(0) = the fraction of samples containing no phage, 

 n = the average number of phage particles per sample, and e 

 = the base of natural logarithms. 



The principle of this method depends on the fact that the 

 chances of a phage particle adsorbing to a bacterium are some- 

 what greater in a broth culture of bacteria than on a plate, 

 owing to more rapid diffusion and longer time available for 

 contact, as well as a high bacterial concentration — 10^/ml. 

 when growth ceases. Once an infected bacterium has lysed, 

 several hundred phage particles are liberated and the chances of 

 detecting the presence of phage by plating are increased. This 

 method obviously cannot overcome a lowered efficiency of 

 plating due to lack of cofactors for adsorption. 



One rather obvious factor which can reduce the efficiency of 

 plating is presence of a considerable proportion of killed bacteria 

 in the culture used for plating. A phage particle adsorbed to a 

 dead bacterium does not multiply or produce a plaque. For 

 this reason only fresh cultures of bacteria should be used for 

 plating. 



