THE INFECTIVE PROCESS 17 



3. Penetration 

 Very little is known about penetration, the second step in the 

 infectious process. It has been assumed that the phage particle 

 must penetrate into the host cell interior, since microscopic stud- 

 ies have quite clearly demonstrated that the phage progeny are 

 formed intracellularly and released on rupture of the host cell 

 membrane. Either the parental phage particle must penetrate 

 the membrane or one must invoke biological action at a dis- 

 tance as great as the membrane thickness. Convincing evidence 

 of penetration was obtained by Hershey and Chase (1952) using 

 T2 coliphage labeled with either radioactive phosphorus or sul- 

 fur. The phosphorus labels the phage nucleic acid while the 

 sulfur labels the sulfur-containing proteins. Hershey and Chase 

 found that if the infected bacteria were subjected to violent agi- 

 tation in a Waring Blendor shortly after infection, a remarkable 

 fractionation of the labels took place. The high shearing forces 

 of the Waring Blendor resulted in the liberation of 75 per cent of 

 the radioactive sulfur but only 1 5 per cent of the phosphorus into 

 the medium, the remaining radioactivity staying with the bac- 

 teria. The treatment did not interfere with the infective proc- 

 ess ; the cells remained capable of yielding phage progeny. The 

 radioactive sulfur-labeled substance which was shaken loose by 

 the Waring Blendor was precipitable by antiphage serum and 

 could be readsorbed to sensitive bacteria, that is, it had the 

 biological specificity of the phage particle. The reasonable con- 

 clusion is that the sulfur-containing proteins of the infecting 

 bacteriophage particles remain on the host cell surface from 

 which they can subsequently be shaken loose, while the nucleic 

 acid of the phage particle penetrates into the host cell. The ex- 

 periment completely alters the problem of phage penetration 

 since it is evident that the intact phage particle does not pene- 

 trate and only certain parts of the particle participate in phage re- 

 production. These parts may well gain entrance by some en- 

 zymatic damage to the host cell membrane produced by viral 

 enzymes, or by viral stimulation of host cell enzymes. Evidence 

 for enzymatic activity on host cell materials is particularly good 



