310 Microbes and You 



Fig. 51. Elie Metchnikoff (1845-1916). {By permission from 

 Introduction to the Bacteria by C. E. Clifton. CopyrigJit, 1950. 

 McGraw-Hill Book Company, Inc.) 



of phagocytic activity, this mechanism is not the only explanation 

 we have to account for immunity. 



The Bacteriophage Theory of d'Herelle 



As minute as bacteria are, they are still subject to parasitic ac- 

 tion by sub-microscopic entities called bacteriophage, or phage 

 for short. The term literally means "bacteria eater," and it is re- 

 garded as being a virus specific for bacteria. After the phage has 

 invaded a susceptible bacterial cell, active multiplication of the 

 virus occurs with an eventual bursting of the host cell and a spill- 

 ing out of the bacteriophage. This dissolving action is called lysis. 

 The lytic action of phage was first reported in 1915 by Twort who 

 was working with a staphylococcus culture growing as a con- 

 taminant in a cowpox vaccine. This original observation was sub- 

 stantiated by d'Herelle in 1917 when he was studying a dysentery 

 culture to which had been added a bacteria-free filtrate of feces 

 obtained from individuals suffering from bacillary dvsentery 



