THE BACTERIOPHAGE AND IMMUNITY 273 



tacked by the bacteriophage does not remain passive. It resists, 

 and is able to overcome the ultramicrobe and to acquire an anti- 

 lytic immunity. All the vicissitudes of the struggle between the 

 animal and the bacterium have their counterpart in the struggle 

 between the parasitizing bacteriophage and the bacterium at- 

 tacked. The resemblance is complete. It is simply a matter of 

 descending a degree in the order of size of the beings involved. 



The existence of the bacteriophage in the intestine of all liv- 

 ing beings, its exiguity which allows it to filter through soils 

 impermeable to bacteria, its vitality and resistance to agents of 

 destruction, explain its extreme diffusion in nature. 



When derived from the organism a single strain of the bac- 

 teriophage is rarely active for but a single bacterial species. 

 Usually it attacks several species at one and the same time, and 

 for each it possesses a separate and variable virulence. There is 

 but one bacteriophage but there is an infinity of strains, each 

 possessing, when taken from the organism, the power of attack- 

 ing a certain number of bacteria. A single strain is variable from 

 time to time, both as to its intensity of action against each bac- 

 terial species, and as to the extent of its action with regard to 

 the number of bacterial species attacked. All combinations of 

 virulence being possible in quantity as in quality, it can be un- 

 derstood in view of the infinite number of possible combinations, 

 that there can exist no two strains of ultramicrobial bacterio- 

 phage which can be exactly alike. 



In a bacterial suspension inoculated with a culture of an active 

 strain of bacteriophage it is not always the latter which prevails. 

 If the bacterium succeeds in acquiring a resistance a selection of 

 more and more resistant bacteria occurs, resulting in the forma- 

 tion of a state of equilibrium between the resistant bacterium and 

 the virulent bacteriophage which then coexist in the medium. 

 A mixed culture results in which the equilibrium is more or less 

 stable but which can be overthrown in favor of the one or the 

 other of the germs there present, according to the circumstances 

 of the moment. 



The acquisition of resistance is accompanied by changes in 

 morphology and in the properties of the bacteria; the bacilli 

 take a coccoid aspect and become surrounded by a capsule. 



