102 BENZERETAL. 



infected the same host. This new entity, the phage-bacterial complex, 

 is then capable of bringing forth the production of several hundred 

 new phage particles. 



There is no good biological analogue for this phenomenon, and 

 small wonder, since we are dealing with intracellular functional organ- 

 ization, which only genetics has tried to probe. In Luria's article, the 

 specific arguments for this new point of view have been set forth in a 

 coherent fashion. In this syllabus, we propose to explain in somewhat 

 greater detail the procedures and facts of phage so as to permit an 

 easier appreciation of the general argument, and as a guide to the 

 literature. 



I. Free Phage 



2. We begin with a description of various features which charac- 

 terize the phages in their extracellular existence, as free phages. Some 

 of these are physical, physico-chemical, and chemical characteristics. 

 Some others are biological characteristics, like host range and activation 

 by cofactors, which show up only in the interaction of the phages with 

 their hosts. They nevertheless serve to distinguish different states in 

 which a phage particle outside the bacterium may be found. 



3. Identification of infective units with physical particles. — There 

 are five different methods for measuring the titer of an infective phage 

 suspension. Three of these methods are biological, viz.: 



(1) Titration by serial dilution to the limit of activity. 



(2) Titration by plaque count. 



It has been shown that the nimiber of plaques is exactly propor- 

 tional to the inverse of the dilution factor over a wide range of con- 

 centration (Ellis and Delbriick, 1939). 



(3) Titration by the killing of bacteria. 



Counting the number of surviving bacteria in an infected culture 

 and applying Poisson's formula one obtains the average number of 

 phage adsorbed per bacterium and thus the total number of phages, 

 once the original number of bacteria is known (Luria and Dulbecco, 

 1949)- 



The fourth and fifth methods are physical. 



(4) Electron microscope counting of particles. 



Certain suspensions of polystyrene latex are made of very small 

 spherical particles all of exactly the same diameter (Backus and Wil- 



