92 THE BACTERIOPHAGE 



heavy suspensions which are inoculated with a filtrate containing 

 a relatively avirulent bacteriophage give after a few hours abso- 

 lutely normal cultures on agar, free of plaques. If serial trans- 

 plants are made of these cultures, inoculations 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 bacterio- 

 phage. This is evidenced by the fact that successive passages 

 from this plaque yield a very active bacteriophage. From whence 

 could this ultramicrobe have so suddenly come? The ultrami- 

 crobe had remained alive within a bacillus, and at a given moment, 

 it overcame the resistance of the latter and multiplied. Its viru- 

 lence being increased, the young germs were able to parasitize 

 the neighboring bacilli and form a colony. Any other explana- 

 tion seems impossible, since, immediately after the inoculation of 

 the bacteriophage, seeding upon agar shows plaques characteristic 

 of the presence of virulent bacteriophagous germs, then these 

 germs completely disappear, the bacteria, however, remaining 

 sensitive to the action of a more active bacteriophage, for perfect 

 lysis is secured if the suspension is inoculated with a trace of a 

 very active culture of the bacteriophage, and finally, the active 

 bacteriophage reappears after a series of subcultures on agar in 

 the course of which all the bacillary cultures have been normal. 

 This germ can only be one of those which had disappeared. The 

 fact, demonstrated by experiment, of the penetration of virulent 

 ultramicrobes into the bacteria, warrants us in thinking that this 

 ultramicrobe (but slightly virulent) has been preserved in a latent 

 living state within the interior of the bacterium. At a given 

 moment the resistance of the bacterium is broken down and in- 

 fection results. 



PRODUCTION OF ANTILYSINS BY BACTERIA 



By its mere presence the bacteriophage is not able to dissolve 

 the bacterium. In the following chapter we will see that it acts 

 through the secretion of lysins which, however, can be isolated 

 free from the ultramicrobe. 



The following experiment shows that the resistance of the 

 bacterium to the action of the bacteriophage, which amounts to 



