86 BACTERIOPHAGES 



ciency of plating is high, the discrepancy must be attributed to 

 aggregation or gross impurity in the phage preparation. This 

 is a rather crude criterion of purity but it suffices to discredit 

 many claims in the literature that pure phage has been pre- 

 pared. Direct examination in the electron microscope can de- 

 tect aggregates of phage as well as contaminating particles with 

 sedimentation properties similar to those of phage particles. 



The analytical centrifuge is useful for detecting impurities with 

 a slower rate of sedimentation than the phage particles. It also 

 gives a measure of the homogeneity of the preparation with re- 

 spect to particle size and density (Putnam, 1953). Electro- 

 phoresis is a useful technique for detecting impurities which have 

 a different mobility in the electric field (Putnam, 1953). Opti- 

 cal analysis of diflE'usion at a phage-buffer boundary has been 

 used also as a test for homogeneity of preparations of phage T6 

 (Putnam, 1953). The specific infectivity of preparations will 

 be low if a fraction of the phage particles are not infective for 

 any reason. Such a preparation might well react as though it 

 were homogeneous by any other test. An inhibitor of phage T2 

 which produces a low specific infectivity has been observed in cer- 

 tain phage lysates but fortunately Sagik (1954) and others have 

 found methods for separating it from the phage. 



An immunological criterion of purity has been used by Cohen 

 and Arbogast (1950a). The most probable impurity in phage 

 purified by diff"erential centrifugation is particulate bacterial 

 debris which should react with antibacterial antibodies to give a 

 precipitate. Cohen obtained such precipitates with purified 

 phage preparations and demonstrated that ribonucleic acid was 

 removed in the precipitate, so that treatment with antiserum 

 was a method of purification as well as a test of purity. Serum 

 proteins could then be removed by centrifugation. The im- 

 munological criterion was used by Kozlofl^ and Putnam (1949) 

 and by Herriott and Barlow (1952). 



The constancy of certain properties of the phage preparation 

 such as infectivity per mg. nitrogen, infectivity per unit of 

 turbidity (extinction at 4,000 A) and infectivity per unit of ab- 



