MECHANISM OF INFECTION 79 



combination with this substance or with all the bactericidal 

 substances in rat serum that bacteria become pathogenic 

 for these animals. 



What proves this is that, in spite of the bactericidal 

 property of their serum, rats do not generally resist a 

 virulent inoculation of anthrax. Another peculiarity, all 

 the more important from this point of view is that when 

 a rat has resisted a primary inoculation it becomes more 

 sensitive to a second inoculation of the same virus. It 

 happens sometimes that an animal can resist three or four 

 successive injections and succumbs only to the fifth. There 

 is here a collection of facts which could not be correlated 

 and which puzzled bacteriologists of the period of 1889-1890. 

 It was not known by what mechanism bacteria might 

 become pathogenic for an animal. Today this mechanism 

 can be explained in the following way: 



Rats in general, especially white rats, resist virulent 

 inoculations of anthrax better than all the other test animals 

 and they owe this relative resistance to the more or less 

 marked bactericidal properties of their plasma. If the dose 

 is not too large the bacteria are destroyed by bacteriolysis 

 and by phagocytes before being able to develop into a more 

 resistant race and the rat recovers. But although the 

 living bacteria may disappear in this way products of 

 bacteriolysis remain in the organism. The products pro- 

 voke the formation of specific antibodies which are not 

 bacteriolytic, but which neutralize a certain quantity of the 

 normal bacteriolytic substance of the organism. The same 

 phenomenon is seen in vitro when the same tube of serum 

 is mixed several times. The second or third mixing will 

 give a culture. 



Something happens here very analogous to what we have 

 seen in the first stage of typhoid infection when bacterial 

 products are incompletely digested in the stomach and 

 intestines, and have penetrated into the blood and into 

 the cells of the intestinal mucosa, and by the formation of 

 an antibody, have rendered the organism more sensitive to 

 bacterial invasion. The bacteria of the second injection 

 will thus resist better and longer the bactericidal^ properties 

 of the organism; their more rapid multiplication will sue- 



