THE BACTEEIOPHAGE AND THE BACTERIUM 93 



an actual immunity in the true sense of the word, is in great part 

 due to the fact that the bacteria secrete antilysins which neutralize 

 the lysins. 



Experiment XXV. A very active strain of bacteriophage, active for 

 B. dysenteriae, is diluted in sterile bouillon, 0. 05 cc. to 10 cc. of the medium, 

 and 0.05 cc. of this dilution is introduced into a second 10 cc. of medium. 

 In addition, a very concentrated suspension of B. dysenteriae is prepared, — 

 a suspension representing a twenty-four-hour slant agar culture in 6 cc. of 

 sterile bouillon. This suspension is inoculated with 0.05 cc. of the second 

 dilution of the bacteriophage culture and incubation at 37°C. for four days 

 follows. At the end of this time it is evident that the suspension is not 

 lysed (because of the too great quantity of bacilli) but when a drop is 

 planted on agar the medium remains sterile. Therefore the bacteriophage 

 has multiplied. This heavy suspension is filtered through a bougie. 



To each of three tubes containing 10 cc. of a weak suspension of dysen- 

 tery bacilli is added — to the first, 0.05 cc. of the filtrate from the heavy 

 suspension, to the second, 0.05 cc. from the first tube, and to the third, 

 0.05 cc. from the second. After an incubation peripd of twenty-four hours 

 it is seen that there is no lysis in tube 1, but lysis is complete in tube 2, 

 while the suspension in tube 3 is still turbid although it clears after forty- 

 eight hours. 



But one conclusion is possible. Since lysis did not take place 

 in the first tube, in spite of the presence of a large number of 

 virulent ultramicrobes, and since it was produced in the other 

 two which received infinitely less, there must have been in the 

 filtrate some substance which inhibited the lytic action of the 

 bacteriophage. This was not manifested in the last two tubes 

 because of the dilution. It is likewise because of dilution that 

 the inhibiting substance did not manifest its action on agar. 

 It has already been noted, in the course of the earlier experiments, 

 that lysis was more often perfect if a suspension was inoculated 

 with a minimal amount of the bacteriophage than if the inocula- 

 tion was massive. The cause for this fact is now clear. 



This experiment is in all respects identical with that described 

 in a preceding paragraph, except that there it was accomplished 

 by introducing a small number of ultramicrobes of low virulence 

 into a large number of bacilli. Under these circumstances the 

 bacteriophage is overcome and destroyed by the bacterium. 

 In this last experiment, on the contrary, we have substituted for 

 the small number of germs of low virulence, ultramicrobes of high 



