The Cardinal Conditions of Infection 77 



number their 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 be achieved by artificial 

 selection so planned as to preserve the more virulent or 

 pathogenic organisms at the same time as the less virulent 

 and more vegetative organisms are eliminated. In cases 

 in which no virulence remains, the experimental manipula- 

 tion of the culture is directed toward gradual immunization 

 of the micro-organisms to the defensive mechanisms of the 

 body of the animal for which the organism is to be made viru- 

 lent. A number of methods are made use of for this purpose. 



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. 



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 con- 

 siderably 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 



