RESISTANCE OF BACTERIA 193 



These two races of the bacteriophage are, however, as different from 

 each other as it is possible for them to be. As I have already stated, one 

 of them, V, is virulent only for the V strain of the Staphylococcus albus. 

 The other race, h, is virulent for all staphylococci, whether they are 

 classed as albus, aureus, or citreus.* 



I have obtained the same results with three races of the Shiga-bacterio- 

 phage. A Shiga bacillus, having become resistant to one of the races was 

 also refractory to the action of the other two. 



These experiments warrant the conclusion that the resistance of a 

 bacterium is by no means limited to the action of a single race of the 

 bacteriophage, ])ut that a bacterium having acquired a resistance against 

 one race of the bacteriophage may manifest a resistance to the action of 

 any other race whatever. It is only in the cases where the resistance 

 acquired is relative that this resistance may be lacking with respect to 

 the action of a bacteriophage of another race. 



It is, indeed, quite impossible to state any fixed rules governing the 

 phenomenon of resistance to the bacteriophage. All that can be said 

 is that usually such and such is true, but that there are many exceptions. 

 For example, in an experiment reported by BaiP" deahng with a strain 

 of B. coll which was naturally resistant to one race of the bacteriophage 

 but susceptible to another, by suitable treatment this strain was induced 

 to acquire a resistance to the last race of bacteriophage, and when this 

 had developed the strain was found to be susceptible to the race for 

 which it previously was resistant. 



It would be just as unwise to generalize from the outcome of this 

 experiment as to draw sweeping conclusions from the fact that under 

 certain circumstances resistance appears to be specific. 



Let us bear clearly in mind, in order to avoid confusion such as has 

 occurred with certain authors, that the natural resistance to a bacterio- 

 phage presented by strains belonging to a heterogeneous species bears no 

 relation to the acquired resistance developing in a susceptible bacterial 

 strain. These two distinct phenomena are comparable to those proc- 

 esses which are designated in immunology by the terms "natural 

 immunity" and "acquired immunity." 



* As we know, an acquired resistance is gradually lost during successive trans- 

 fers. In this experiment, the resistance of the V strain of the staphylococcus to 

 bacteriophage v is lost after 7 transfers; resistance to bacteriophage h is lost only 

 after 19. 



