BACTERIOPHAGE AS A MODEL OF HOST-VIRUS RELATIONSHIP 199 



During the vegetative phase of a virus, during the rapid synthesis of viral 

 proteins and nucleic acids, a more or less important part of the host's 

 metabolism is diverted toward the synthesis of foreign structures. Whether 

 fatal or not, this spoliation might be considered as a disease. The synthesis of 

 a prophage, however, diverts such a small fraction of the bacterial metabo- 

 lism that it can hardly be considered as a malady, A prophage is not patho- 

 genic, in the sense that it does not produce a recognizable disease. If the 

 vegetative phase of the life cycle, that is to say the production of bacterio- 

 phage particles, were itself non-pathogenic, bacteriophage could be visualized 

 as a part of the life cycle of the bacterium, as a sort of aberrant gamete. 



When a process ends with the death of a cell or of an organism, patho- 

 genicity cannot be questioned. It is, however, manifest that any disease 

 might be subclinical, and that the definition of normalcy and health is a 

 most arduous task. The viruses which have been studied so far are agents, or 

 potential agents, of diseases. In order to detect, a virus one utilizes, among 

 other characters, its pathogenic activity either on a sensitive organism or on a 

 sensitive cell. An entity completely devoid of recognizable pathogenic 

 activity would be difficult to identify as a virus. And yet, a particle could 

 exist, having the life cycle of a virus, including an crganized infectious 

 particle of definite size and structure, and be devoid of pathogenicity. 



Moreover, pathogenicity is only a potential feature of viruses. The 

 expression of the pathogenicity of prophage depends on the probability of an 

 unknown internal metabolic process of the lysogenic bacterium. The expres- 

 sion of the pathogenicity of the virus sensitizing Drosophila to CO 2 depends 

 on an environmental factor, a high CO 2 tension. In one case, expression 

 corresponds to the transition from prophage to the vegetative phase of the 

 cycle. In the other, it is only the revealing of a certain state of the infected 

 Drosophila. The probabiHty for a virus to express its potential pathogenicity 

 could be so low that it would not be detectable. It is felt, therefore, that 

 pathogenicity, despite its operational importance, should be and could be 

 excluded from the definition of viruses and even of infection. A virus would 

 be an entity ivith an organized infectious phase, containing proteins, possessing 

 one type of nucleic acid, reproduced from its genetic material, unable to grow and 

 to undergo binary fission, and devoid of a Lipmann system. Infectivity of virus 

 is defined as the ability to introduce its genetic material, that is to say a specific 

 information, into a cell. 



These definitions are satisfactory in a way, although they do not mention 

 one of the important characteristics of viruses, namely, their power to 

 modify their host. 



When considering a lysogenic bacterium and trying to understand the 

 nature of the relationship between the bacterium and the prophage, one 

 realizes that the prophage is not "neutral," It confers on the lysogenic 



