194 THE BACTERIOPHAGE AND ITS BEHAVIOR 



4. THE ACQUISITION OF RESISTANCE 



How can this acquisition of immunity by a bacterium be explained? 

 Numerous experiments have shown that if a certain quantity of a 

 slightly active suspension of a bacteriophage is introduced into a rela- 

 tively heavy (1000 to 2000 milHon per cubic centimeter) suspension of 

 bacilli, the corpuscles, readily demonstrated at first by the presence of 

 plaques on plantings on agar, disappear from the medium after an inter- 

 val of time varying from one hour to two or three days, and that they 

 can not later be demonstrated. Subcultures give normal cultures of 

 bacteria. On the other hand, we have seen that with a very virulent 

 bacteriophage the corpuscles disappear from the fluid between ten and 

 twenty minutes after introduction into a suspension, but that they reap- 

 pear in about twenty times as great a number in from one to one and a 

 half hours later — they have multipHed within the interior of the bac- 

 teria. In the case of a bacteriophage of low virulence it seems, there- 

 fore, that penetration of the bacteria takes place but that multipHca- 

 tion can not be effected. The bacterium resists and the corpuscle is 

 actually destroyed in vivo. These parasitized bacteria which "recover" 

 acquire by this an immunity. 



Another fact has been sometimes observed which shows that certain 

 bacteria are able to become "carriers." As has been said, heavy sus- 

 pensions which are inoculated with a filtrate containing a relatively 

 avirulent bacteriophage give after a few hours absolutely normal cul- 

 tures on agar, free of plaques. If serial transplants are made of these 

 cultures, the plantings being made in such a manner as to yield an even 

 layer of growth, it sometimes happens that after a certain number of 

 transplants, two to four, a very definite plaque appears, which is indeed 

 a colony of the bacteriophage corpuscles. This is evidenced by the 

 fact that successive passages from this plaque yield a very active bac- 

 teriophage. From where could this corpuscle have so suddenly come? 

 The corpuscle had remained alive within a bacillus, and at a given 

 moment, it overcame the resistance of the latter and multiplied. Its 

 virulence being increased, the young corpuscles were able to parasitize 

 the neighboring bacilli and form a colony. Any other explanation seems 

 impossible, since, immediately after the inoculation of the bacterio- 

 phage, seeding upon agar shows plaques characteristic of the presence 

 of virulent bacteriophagous corpuscles, then these corpuscles completely 

 disappear, the bacteria ,however, remaining sensitive to the action of a 

 more active bacteriophage, for perfect dissolution is secured if the suspen- 



