32 BACTERIOPHAGES 



should be considered whenever plating methods are unsatis- 

 factory. 



One or another of these assay methods should suffice for most 

 purposes. A few other methods have been used for special pur- 

 poses. The use of the electron microscope to determine the total 

 number of morphologically typical phage particles in a known 

 volume of suspension has been mentioned above. A method that 

 involves the ability of phage particles to kill the host cell on 

 adsorption is particularly suitable to the assay of phage inac- 

 tivated by ultraviolet light. On adsorption the phage particles 

 are distributed over the host cell population in accordance with 

 the Poisson distribution. The viable bacterial population is 

 accurately determined before and after phage adsorption. The 

 proportion of the bacterial population that survives is equal to 

 ^~" where n is the mean number of lethal particles adsorbed per 

 bacterium. Then n times the initial number of bacteria per 

 unit volume gives the number of phage particles, per unit volume, 

 adsorbed and capable of killing. Conditions should be such 

 that most of the phage particles present are adsorbed to bacteria 

 and the value of n should be greater than one (Luria and 

 Dulbecco, 1949). 



4. Intracellular Phage 



Mature phage particles present in yet unlysed bacteria present 

 a special assay problem. An infected bacterium if plated be- 

 fore bursting will form only a single plaque regardless of how 

 many mature phage particles it may produce. However, the 

 number of mature phage particles present in the bacterium at 

 any time during the latent period may be determined by inter- 

 rupting phage development by chilling or by chemical agents, 

 and liberating the mature phage by breaking open the infected 

 bacteria by biological, enzymatic, or physical means (Doermann, 

 1952; Anderson and l])oermann, 1952a). The average number 

 of mature phage particles per bacterium liberated by premature 

 lysis may then be determined by plaque counting methods. 



A peculiar biochemical feature of phage T2 and its relatives 



