98 THE BACTERIOPHAGE 



1918, and 1919 to a large number of passages, somewhat more 

 than 1200, always with the dysentery bacillus. Nevertheless, 

 early in 1920 experiment showed that it had an average virulence 

 for B. coli and a very low activity for B. typhosus. 



The action on the typhoid bacillus of a bacteriophage which 

 had received more than a thousand passages with B. dysenteriae 

 is evidently weak. This can be demonstrated by spreading on 

 agar, a procedure which permits the formation of characteristic 

 plaques. If we introduce into a tube of bouillon about ten drops 

 of an anti-dysentery bacteriophage and then a small amount of 

 typhoid culture, we secure, after incubation for eighteen to twenty- 

 four hours, a culture of B. typhosus which appears normal, but 

 if a drop is seeded upon agar a few plaques are obtained. 



The following observation, made by G. Eliava, is of the same 

 nature, but more typical, for there is a crossed reaction on bac- 

 teria but remotely related. A strain of bacteriophage isolated 

 from the pus of an abscess (we will see that under certain cir- 

 cumstances the intestinal bacteriophage may pass into the cir- 

 culation) and very active against Staphylococcus aureus, appeared, 

 even after a series of more than 100 passages with the staphylo- 

 coccus, to be endowed with a degree of activity for the dysentery 

 bacillus. 



Experiment XXVIII. Ten cubic centimeters of a Shiga suspension is 

 inoculated with 0.25 cc. of a filtered anti-staphylococcus bacteriophage. 

 When plated immediately on agar a normal culture of B. dysenteriae 

 develops. After incubation at 37°C. for twenty-four hours, 0. 02 cc. of this 

 suspension plated on agar gives 4 plaques. This virulence for B. dysen- 

 teriae was increased by successive transfers in association with this 

 organism. 



Such experiments allow us to penetrate further into the phe- 

 nomenon of virulence of the bacteriophage. 



We have introduced into the suspensions, either of B. typhosus 

 or of B. dysenteriae, several hundred millions of ultramicrobes, 

 all virulent for the Shiga bacillus in the first case, for the staphy- 

 lococcus in the second. Each plaque on the agar represents a 

 colony derived from a virulent ultramicrobe; virulent in one case 

 for the typhoid bacillus, in the other for B. dysenteriae. We have 

 also seen that of the several hundred millions of ultramicrobes 



