THE INFECTIVE PROCESS 19 



tinues (Cohen, 1949). Synthesis of ribose nucleic acid almost 

 stops but protein synthesis continues. The synthesis of bacterial 

 deoxyribose nucleic acid (DNA) stops but after a short lag the 

 synthesis of phage DNA begins. The synthesis of phage protein 

 and DNA then continues until bacterial lysis occurs (Hershey, 

 Garen, Fraser, and Hudis, 1954). 



A third approach to a knowledge of intracellular multiplica- 

 tion consists in stopping virus synthesis by chilling or by meta- 

 bolic poisons such as cyanide at appropriate intervals in the latent 

 period. The infected cells are then lysed by secondary means 

 such as a second phage or sonic vibrations and the lysates are 

 assayed to determine the mean number of mature phage par- 

 ticles present per infected host cell. By such means it has been 

 found that no mature, infectious phage particles are present 

 intracellularly until half-way through the latent period. Then 

 the number of mature phage particles increases as a linear func- 

 tion of time until shortly before lysis (Doermann, 1952). 



In such circumstances it is useful to distinguish between two 

 kinds of phage particles, mature or infective particles and im- 

 mature, noninfective, vegetative particles. The mature phage 

 particle is the typical extracellular stage which is assayed by its 

 plaque forming ability. The immature or vegetative phage 

 particle is the intracellular stage, undergoing multiplication, 

 potentially capable of producing mature phage particles but not 

 detectable by the plaque count method because it is noninfec- 

 tious. 



The following picture of phage multiplication emerges from 

 these observations: 



7. The phage particle adsorbs to the host cell surface. 



2. Certain reproductively essential components of the phage 

 penetrate to the cell interior, expendable portions being dis- 

 carded at the ceil surface. The phage particle thus changes from 

 the mature to the vegetative condition. 



3. The host cell stops dividing, certain synthetic reactions 

 stop, and extensive cytological changes occur. 



4\. Synthesis of phage protein and phage DNA start, and the 



