THE CLASSIFICATION OF BACTERIOPHAGES 343 



varies in reactivity, so that not all collisions of antibody and phage in the neutraUzation 

 mixtures are fruitful (Kalmanson, Hershey and Bronfenbrenner 1942). Even with fuUy 

 neutraUzed phage, there is no evidence that union of antibody with phage alters it per- 

 manently, even after prolonged contact (Kalmanson, Hershey and Bronfenbrenner 1942). 

 After 56 days' contact with antibody at 37° C, Kalmanson and Bronfenbrenner (1943) 

 were able to reactivate a just-neutraUzed phage by digesting the inactive phage with 

 papain. The digestion apparently removed only a portion of the combined antibody 

 molecules, but m doing so made the bacterial receptors fuUy accessible. 



The relations of phage-inactivation by antisera and by bacterial antigens are 

 summarized in Fig. 54, which is modified from a scheme devised by Delbriick (1942). 



Reaction Result 



B + P — > BP Adsorption of phage to bacteria ; lysis. 



b + P — >■ bP Adsorption of phage to free bacterial antigen ; inactivation of phage. 



bP + B No lysis, or delayed lysis, according to degree of phage-inactivation. 



B + i? — > B^ Antigen-antibody union ; agglutination of bacteria. 



b + i3 — > hji Antigen-antibody union ; precipitation of bacterial antigen. 



P + 77 — >- Ptt Antigen-antibody union ; inactivation of phage ; (agglutination of large- 



particle phages). 



Ptt + B No lysis, or delayed lysis, according to degree of phage-inactivation. 



Ptt + b No adsorption of phage to free bacterial antigen. 



BP + 77 — > BPtt Antigen-antibody union ; agglutination of phage-coated bacteria by 



antiphage serum. 



Fig. 54. The reactions of phage-inactivation by bacterial antigens and specific 



antiphage sera. 

 B = bacterium. 



b = antigenic component of bacterium associated with specific adsorption of phage. 

 j8 = antibody to b. 

 P = phage with affinity for B. 

 77 = antibody to P. 



Other Methods of Phage Differentiation, and the Identification and Classification of 

 Phages. 



The phage plaques that have so frequently been referred to in preceding sections 

 have characters that are of great value in the differentiation of one phage from 

 another. For any one phage, acting on any one sensitive bacterium, the plaques 

 formed on an agar plate are usually closely similar, both in size and in form. But 

 different phages acting on the same bacterium may give plaques that differ sharply 

 and characteristically from one another. 



To take first the character of plaque size, the existence of large-plaque-forming 

 phages and small-plaque-forming phages has long been recognized, and used as a 

 basis for differentiation (see Bail 1923). ELford and Andrewes (1932) were able 

 to show that there is an inverse relation between plaque size and phage size. The 

 small phages give big plaques, and vice versa. It is an obvious assumption that the 

 small phages can diffuse more readily through the surface bacterial growth, and so 

 extend over a larger radius. The probability of this assumption is increased by 

 the demonstration by Andrewes and Elford (19336) that a large-plaque-forming 

 phage, incompletely neutralized by an antiphage serum and probably aggregated 

 into small clumps, gives small instead of large plaques. 



But size is not the only character that differentiates one phage plaque from 

 another ; there are often marked differences in the edge of the circular clearings. 



