RESISTANCE OF BACTERIA 239 



"law of reaction." Bacteria attacked by bacteriophage corpuscles 

 react and may acquire a resistance such as to afford a true immunity. 

 According to the virulence of the corpuscles, to the tendency to resist 

 on the part of the bacteria, and to the conditions of the moment, the 

 one or the other of the two antagonists finally predominates. If it is 

 the bacteriophage which is victorious the medium in which bacteri- 

 ophagy takes place remains clear indefinitely, it being a pure suspension 

 of bacteriophage corpuscles. If the bacteria succeed in acquiring an 

 immunity the medium again becomes cloudy, as a result of the devel- 

 opment of resistant bacteria. Here there is a secondary culture 

 (d'Herelle^i^). 



2. The secondary cultures result from a phenomenon of selection, not 

 of bacteria naturally endowed with resistance but of certain bacteria 

 which possess a faculty for the acquisition of resistance. In a given 

 suspension the number of bacteria capable of acquiring resistance is the 

 greater as the virulence of the bacteriophage is the less (d'Herelle^^^). 



3. The resistance acquired by a bacterium does not manifest itself 

 solely toward the race of the bacteriophage in contact with which the 

 resistance has been acquired but against other races as well. A bac- 

 terium which has once become refractory to the action of one race of 

 bacteriophage is refractory to the action of all other races. 



4. Experiment shows that the protoplasm of bacteria which have 

 acquired a resistance is able to destroy bacteriophage corpuscles 

 (d'Herelle,^-^ Flu-'^). This experimentally demonstrated fact renders 

 plausible the hypothesis that the acquisition of resistance by a bac- 

 terium takes place after "recovery," that is to say, as the result of the 

 destruction of the corpuscles within the protoplasm of this bacterium. 



5. Bacteriophage corpuscles which are enhanced by passages with 

 susceptible bacteria are attenuated during the process of the acquisition 

 of resistance by the bacteria. 



6. If one purifies a secondary culture, that is to say, a culture containing 

 bacteria endowed with an acquired resistance and virulent bacteriophage 

 corpuscles, either by the use of an antibacteriophage serum (Bordet 

 and Ciuca^") or by the fishing of ultrapure colonies (Eliava and 

 Pozerski^^'^) it will be found that in the course of a series of transfers on 

 agar the acquired resistance gradually diminishes, and then disappears, 

 and at this time the bacteria have again become susceptible (Bordet 

 and Ciuca«2)^ 



7. When planted upon agar secondary cultures give a variable result, 

 depending upon the virulence of the bacteriophage. With very viru- 



