226 THE BACTERIOPHAGE 



The results are thus identical with those secured at Bac Lieu, 

 although the virulence seems to be somewhat lower, but this 

 can only be of relative importance since in the present case each 

 specimen was composed of the excreta taken from several rats; 

 the results then, indicate only an average. 



Is the bacteriophage present in all of the rats of an infected 

 region or only in a certain number? At Phantiet I collected the 

 excrement of six young rats, according to their weight, aged from 

 three to four weeks. Examination showed that four of the speci- 

 mens contained a bacteriophage active for B. pestis (+) while 

 two did not. These last two animals were therefore susceptible 

 to plague. 



From the results given above one may conclude that, as for 

 avian typhosis and for barbone, the cause of the resistance against 

 B. pestis is the presence in the organism of a bacteriophage pos- 

 sessing a virulence for this bacillus. 



How is the adaptation effected in the case of B. pestis? At 

 different times it has been noted that the bacillus has been found 

 in the intestinal contents of victims of plague. Thus, it is pos- 

 sible for them to be disseminated by the feces throughout 

 the external world where they may again be ingested. The 

 bodies of dead rats constitute another mode of dissemination. 

 These bodies are often devoured by the surviving rats and this 

 extends the infection. In those animals which resist and which 

 are infected the intestinal bacteriophage is maintained virulent 

 for the pathogenic bacillus. But observation and direct experi- 

 mentation have shown us that a bacteriophage is only possessed 

 of a virulence for a bacterium when the ingestions of this bac- 

 terium are frequent. The permanence of the virulence of the 

 intestinal bacteriophage of the rat against the plague bacillus 

 indicates the persistence of this bacillus in the external world, 

 at least for several months after the last human case has taken 

 place. Moreover, the -revival of the epidemic each year in cer- 

 tain localities, Bac Lieu for example, shows that it can not be 

 otherwise. 10 



10 Demonstration of the presence of a bacteriophage active for B. pestis 

 in the rats of a locality would in certain cases be very useful for it would 

 indicate the presence of the bacillus in the exterior world and the possi- 



