194 A. LWOFF 



bacterial component is important; however, it appears immediately that 

 bacteriophage is not an ordinary bacterial component. Our task is to find out 

 whether it is possible to discriminate between bacteriophage and typical 

 organelles. Two main characters have to be considered, namely, patho- 

 genicity and infectivity. 



Pathogenicity, the power to ehcit a disease, is not a characteristic trait of 

 bacteriophage. Bacteria may possess lethal genes which are the potential 

 agents of fatal diseases. Pathogenicity is therefore not a character allowing 

 one to discriminate between bacteriophage and cellular organelles. Let us 

 now consider infectivity. 



Infectivity is generally defined as "having the quality or power of com- 

 municating disease by infection" and infection is generally defined as "the 

 communication of a disease." Infection and disease are in fact often associated 

 but this is not always the case. When a bacteriophage injects its genetic 

 material into a bacterium, the bacterium may become lysogenic. It remains 

 healthy and multiplies. It is nevertheless clear that the bacterium has been 

 "infected." Infection in this case is the introduction into an organism of an 

 entity able to reproduce true to type. According to this definition, the transform- 

 ing principle should be considered as infectious. The introduction of patho- 

 genicity in the definition of infection would not solve our problem, for one can 

 conceive of a lethal gene introduced in lieu of "normal" transforming principle 

 and producing a disease. Yet everyone has the feeling that the introduction 

 from outside of a lethal gene is something radically different from the 

 introduction of the genetic material of a bacteriophage. We need to find out 

 how transformation can be excluded from infectious processes. 



During the vegetative phase of the life cycle of a bacteriophage, organized 

 infectious particles containing proteins and nucleic acid are produced. The 

 differences between a disease started by a lethal gene and a disease started 

 by the genetic material of a bacteriophage are manifold. 



1. So far as we know, the lethal gene is not reproduced during the disease, 

 which consists mainly in a disturbed metabolic process that might be the 

 synthesis of an abnormal structure. It is not accompanied by the multi- 

 plication of the lethal gene. During the vegetative phase of the phage cycle, 

 abnormal structures such as proteins are also produced but the genetic 

 material of the phage is multiplied. 



2. No infective particles are produced during the disease initiated by a 

 lethal gene, whereas infective particles are formed during the vegetative phase 

 of the cycle of a bacteriophage. These are "organized" particles, composed 

 essentially of a nucleic acid and proteins, possessing a definite size and 

 structure. 



From all we know concerning the life cycle of a virus, infectivity of a virus 

 is the ability to introduce its genetic material into a cell. Accordingly, an 



