100 THE BACTERIOPHAGE 



for a very long time, probably indefinitely, the capacity to attack 

 a bacterium of the enteric group, and this in spite of very many 

 passages in conjunction with the bacterium accidentally attacked. 

 On the contrary, a bacteriophage possessing, when derived from 

 the body, a virulence for a bacterium accidentally attacked, 

 loses more or less rapidly this virulence if it is maintained in 

 vitro with a bacterium normally attacked, that is to say, at the 

 expense of an organism of the colon-typhoid-paratyphoid-dys- 

 entery group. 



It may be objected that all these facts may be interpreted, not 

 as a persistence of a latent virulence but as a persistence, through 

 the course of successive passages, of several species of the bac- 

 teriophage. In other words, there has been a " contamination" 

 of the anti-dysentery bacteriophage by an anti-typhoid bacterio- 

 phagous ultramicrobe in the first case cited, and a " contamina- 

 tion" of the anti-staphylococcus bacteriophage by an anti-dys- 

 entery bacteriophage in the second. 



Both experimentation and mathematical reasoning show that 

 such an interpretation is false. 



In the first example cited, it has been shown that the number 

 of ultramicrobes virulent for B. typhosus did not vary during the 

 course of the passages. The number of plaques obtained on agar 

 by inoculation of a suspension of typhoid bacilli inoculated with 

 the anti-dysentery bacteriophage is essentially the same, although 

 the bacteriophage has been subjected to fifty, one hundred, five 

 hundred, or a thousand passages at the expense of B. dysenteriae. 

 If the ultramicrobes capable of attacking the typhoid bacillus 

 were an "impurity" their number should diminish gradually in 

 the course of the transfers, each passage being a dilution, and 

 they should quickly disappear. 



If one calculates the extent of the dilution, after a thousand 

 passages, of the filtrate which served as the original inoculum 

 for the bacteriophage in question, a figure of such magnitude is 

 obtained that a persistence of anti-typhoid bacteriophagous germs, 

 throughout the series of successive dilutions, is mathematically 

 impossible. One can calculate readily up to the thousandth 

 passage (each passage consisting of the inoculation of 0.001 cc. 

 of filtrate into ten cc. of bouillon). The value of the dilution in 



