The Cardinal Conditions of Infection 75 



number their less vegetative but more pathogenic fellows. 

 Each time the culture is transplanted, more of the vegetative 

 and fewer of the pathogenic forms are carried over, until 

 after the organism is said to be accustomed to its new 

 environment, and grows readily upon the artificial media, 

 it is found that the pathogenic organisms have been largely 

 or entirely eliminated and the vegetative forms alone remain. 



Increase of virulence can rarely be effected without 

 recourse to animal inoculations, or some modification of the 

 culture method in which animal juices play an important 

 part. It can be explained in the same way as decrease of 

 virulence, by supposing that the method adopted for culture 

 is adapted to the needs of the pathogenic members of the 

 family and less so to the more distinctly vegetative forms. 



Three means of increasing the virulence of bacteria are 

 sufficiently important to merit the attention of the student. 



1. Passage through animals. Except in cases where the 

 virulence of the micro-organism is invariable, it is usually 

 observed that the transplantation of the organism from 

 animal to animal without intermediate culture in vitro 

 greatly augments its pathogenic power. Of course, this 

 artificially selects each time those members of the bacterial 

 family best qualified for development in the animal body, 

 eliminating the others, and the virulence correspondingly 

 increases. 



The increase in virulence thus brought about is, however, 

 not so much an increase in the general pathogenic power of 

 the organism for all animals, as in pathogenic activity toward 

 the particular animal or kind of animal used in the experi- 

 ments. Thus, in general, the passage of bacteria through 

 mice increases their virulence for mice, but not necessarily for 

 cats or horses; passage through rabbits, the virulence for 

 rabbits, but not necessarily for dogs or pigeons, etc. 



This specific character of the virulence is well accounted 

 for by the " lateral-chain theory of immunity," where it will 

 again be considered. 



2. The use of collodion sacs. When cultures of bacteria 

 are enclosed in collodion sacs and placed in the abdominal 

 or other body cavities of animals, and kept in this manner 

 through successive generations, the virulence is usually 

 considerably increased. This is one of the favorite methods 

 used by the French investigators. It keeps the bacteria in 

 constant contact with the slightly modified body juices of 



