Chapter V 



Bacteriophage as a Model of Host- Virus Relationship 



Andre Lwoff 

 Department of Microbial Physiology, Pasteur Institute, Paris, France 



I. Introduction 187 



II. The Bacteriophage 189 



A. The Vegetative Phase 189 



B. The Pro viral Phase and the Lysogenic Bacterium 193 



III. The Origin of Bacteriophage 191 



IV. Is Bacteriophage an Organism? 192 



V. Bacteriophage and Cellular Organelles 193 



VI. Bacteriophage as a Virus 195 



VII. Are Viral Diseases Always Infectious? 197 



VQI. Remarks on the Pathogenicity of Viruses 198 



IX. Conclusions 200 



References 201 



I. Introduction 



Between 1860 and 1915, numerous bacteriologists had handled a consider- 

 able number of bacterial cultures. Many of them had certainly observed the 

 disappearance of bacterial population or colonies. Among them was d'Herelle, 

 who, around 1910, described the "taches vierges" of the colonies of Cocco- 

 bacillus acridiorum. A few years later, Twort observed the vitreous degenera- 

 tion of some colonies of a Micrococcus. He had, however, the idea that bacterial 

 lysis could be an "acute infectious disease." He succeeded in transferrmg the 

 active principle from vitreous to normal colonies, showed that it was filterable, 

 that it multiphed and reproduced true to type, and that the number of lesions 

 was inversely proportional to the dilution. In his classic paper, which appeared 

 in 1915, Twort discussed the possible nature of the infectious agent: it could be 

 an ultravirus, or a small parasite reproducing at the expense of the bacterium, 

 or a phase of the life cycle of the micrococcus, or an autocatalytic enzyme, or 

 a primitive form of life. 



The existence of acute infectious diseases of bacteria was confirmed two 

 years later by d'Herelle, who named the agent bacteriophage. This was the 

 origin of extensive investigations. Bacteriophage is now the virus about which 

 we possess the most extensive information, and is an excellent model for the 

 understanding of viruses. 



Until the end of the nineteenth century, the history of viral diseases is just 

 a part of the history of infectious diseases. In about 2500 B.C., the Chinese 



187 



