BACTERIOPHAGE AS A MODEL OF HOST-VIRUS RELATIONSHIP 



189 



infectivity and of pathogenicity. Some virologists consider viruses as organ- 

 isms, others, as cellular organelles, others, as molecules. This diversity 

 exemplifies the difficulty of drawing a border line between viruses and other 

 entities. The task is hard, but has to be performed if we want to gain a clear 

 picture of the virus. 



II. The Bacteriophage 



A bacteriophage is a particle built up of nucleic acid and protein and 

 provided with a tail. When the tip of the tail meets a receptive bacterium, 

 it attaches itself to the bacterial wall. Then the genetic material of the 

 phage, mostly nucleic acid, passes into the bacterial cytoplasm. The infected 

 bacterium can evolve in two ways: (1) The vegetative phase is initiated, 

 bacteriophage particles are produced, and the bacterium dies. (2) The 

 genetic material does not enter the vegetative phase. It is converted into a 

 prophage. The bacterium remains alive and is now lysogenic. Let us consider 

 more closely these two types of bacterial responses (Fig. 1). 



-C3< 



Loss of prophoge 



rCZD^ 



nri non^lysoqenic bflD (ZED lysogenic b. 



.ysogenizatiory 



'CD 



T 



^rj^' 



.-i'A 



♦ r.^^ 



Productive p-^^js' 

 infection ' — -* 



T 



Induction 



g 



■^ciD 



^X»"" lapa 



Fig. 1. The life cycle of a temperate bacteriophage. The prophage is represented by a 

 dash; b = bacterium. 



A. The Vegetative Phase 



The vegetative phase is characterized by the multiplication of the genetic 

 material of the phage, by the synthesis of phage proteins, and by the morpho- 

 genesis of phage particles. Each infected bacterium produces some 100 

 particles which are liberated as a result of bacterial lysis. 



How is phage reproduced during the vegetative phase? It is admitted that 

 the phage constituents, the amino acids, the purine and pyrimidine bases, and 



