76 MECHANISM OF INFECTION 



tures are finally obtained, of which a small dose will kill 

 by ingestion field and white mice in four to six days and 

 rats in six to twelve days. 



These cultures have been preserved in sealed ampoules for 

 one to ten years and tested as to their virulence once to 

 twice a year. They have always preserved their virulence 

 and have even become a little more pathogenic for mice, 

 but at the same time they lose their virulence for rats, so 

 that after ten years they have become completely avirulent 

 for these animals. By passing a culture, virulent for the two 

 species, through ordinary broth, the virulence for the rats 

 remains constant for one or two years, but finally diminishes 

 and almost disappears at the end of a longer or shorter 

 time. Thus we conclude: 



1. That the bacterial substance virulent for mice is 

 different from that which is virulent for rats. 



2. That the bacteria can produce, augment and lose 

 this specific pathogenicity substance. 



3. That this property is progressively acquired by being 

 nourished with the "substance rat" and that it is lost 

 when not nourished in this way. 



By analyzing these facts we are obliged to assume that 

 in order to nourish itself with the "substance rat," the 

 bacteria has been obliged to learn to fix this substance by 

 a special chemical affinity, and the fact that the bacteria 

 can multiply this fixation substance, even when trans- 

 planted into a non-specific nutritive medium, obliges us to 

 assume that the fixation is intracellular. 



Thus, in the last analysis, the substance of the bacteria 

 acquires a specific affinity for the "substance rat" and it is, 

 thanks to this acquired affinity, that the bacteria, or more 

 exactly, its own specific substance, can fix and digest the 

 "substance rat" and render it assimilable. 



But it is evident that affinities ought to be always recipro- 

 cal and that in consequence the fixing substance of the 

 bacteria, freed by bacteriolysis, can fix itself by the same 

 affinity and produce reactions of the same nature in an extra- 

 or intracellular substance of the rat when it finds itself in 

 the organism of this animal. 



