ENUMERATION OF BACTERIOPHAGE PARTICLES 31 



can be computed (Haldane, 1939). Phage assays by this method 

 should check closely with assays by the agar layer method if both 

 media are optimal for phage multiplication. This is one way 

 of defining the relative efficiency of plating. 



3. Measurement of Lysis Time 



A kinetic method of phage assay was developed by Krueger 

 (1930), for use with rapidly lysing phages, which gives precise 

 and reproducible results within certain limits. The procedure 

 is to add appropriate dilutions of phage to tubes containing 

 standard suspensions of actively growing host cells, and to de- 

 termine the length of time required for lysis to reduce the bac- 

 terial turbidity to an arbitrary end-point. The time of lysis is 

 inversely proportional to the logarithm of the initial phage con- 

 centration over a considerable range. A standard phage prep- 

 aration is assayed in parallel with the unknown and the activity 

 of the unknown is expressed in terms of an arbitrary unitage de- 

 fined for the standard. One defect of the method is that it does 

 not give the assay in terms of infectious phage particles but only 

 in amounts relative to the standard. This defect could be reme- 

 died by determining the content of infective particles in the stand- 

 ard phage preparation by one of the methods discussed above. 

 Another defect is that the method is applicable only to free phage 

 particles. A suspension of phage after adsorption to host cells 

 will assay higher than the same amount of free phage because the 

 time required for adsorption is not included in the observed 

 lysis time for the adsorbed phage but is included in the case of 

 the standard phage suspension. Therefore the application of 

 the kinetic assay method to the study of phage adsorption 

 (Krueger, 1931) and intracellular multiplication (Krueger and 

 Northrop, 1930) has led to erroneous interpretations of the data. 

 The method has been applied successfully to the assay of phages 

 for anaerobic bacteria when for technical reasons the plaque 

 count method did not giv^e satisfactory results (Gold and Wat- 

 son, 1950). The method may well have other applications and 



