96 Infection 



of the pathogenic forms are carried over, until after the 

 organism is accustomed to its new environment, and grows 

 readily upon the artificial media, it is found that the patho- 

 genic organisms have been largely or entirely eliminated and 

 the vegetative forms alone retained. 



Increase of virulence can be achieved by artificial 

 selection so planned as to preserve the more virulent or 

 pathogenic organisms at the same time that 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 

 virulent. 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 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 toward the particular animal 

 or kind of animal used in the experiments. 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 

 the animal, which transfuse through the collodion, and thus 

 impedes the development of such organisms as are not able 



