28 BACTERIOPHAGES 



will produce a plaque. Although the plaque count is a good 

 relative assay method, it will not give the absolute number of 

 phage particles present in the inoculum. 



If the same phage preparation is assayed under different en- 

 vironmental conditions it is immediately obvious that the plaque 

 count has no absolute significance. A change in salt concentra- 

 tion can change the assay of coliphage T2 1,000-fold, while omis- 

 sion of tryptophan can reduce the plaque count of certain 

 strains of phage T4 by a factor of 10"*. Assaying the same phage 

 preparation on a series of different susceptible bacterial strains 

 will often result in a different assay value with each host. Such 

 observations led to the concept of efficiency of plating (Ellis and 

 Delbruck, 1939). 



The relative efficiency of plating (EOP) can be defined as the 

 plaque count obtained under a given set of conditions relative 

 to the plaque count under standard conditions; for instance, 

 the EOP of phage T2 on E. coli strain B/6 is 0.8 of that on strain 

 B. For such comparative purposes it is reasonable to use as 

 standard those conditions that are found to give the highest 

 plaque count. However, it is unwise to assume that such 

 standard conditions enable one to count all the potentially in- 

 fective phage particles present in the inoculum. 



The absolute efficiency of plating may be defined as the plaque 

 count relative to the total number of phage particles present in 

 the sample. At present there are few practicable methods for 

 determining the absolute efficiency of plating. One method 

 compares the plaque count obtained per unit volume of phage 

 preparation with an electron microscopic count. Luria, Wil- 

 liams, and Backus (1951) made measurements of this kind. 

 The phage was suspended in a solution containing a known con- 

 centration of polystyrene latex particles, and the mixture was 

 sprayed upon specimen screens for electron microscopy. All 

 morphologically characteristic phage particles and all latex 

 particles were counted in a number of fully visible droplet pat- 

 terns. The latex particle count was used to determine the total 

 volume of counted droplets, and the phage particle count then 



