The Cardinal Conditions of Infection 81 



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 can be explained by the 

 "lateral-chain theory of immunity," where it will again be con- 

 sidered. 



The Use of Collodion Sacs. When cultures of bacteria are en- 

 closed 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 the animal, which transfuse through the collodion, 

 and thus impedes the development of such organisms as are not able 

 to endure their injurious influences. Thus it becomes only another 

 way of carrying on an artificial selection of those members of the 

 bacterial family that can endure, and eliminating those that cannot 

 endure the defensive agencies of those juices with which the organ- 

 isms come in contact.* 



The addition of animal fluids to the culture-media sometimes 

 enables the investigator to increase, and usually enables him to 

 maintain, the virulence of bacteria. A series of generations in 

 gradually increasing concentrations of the body fluid should be 

 employed, until the organism becomes thoroughly accustomed to it. 



In some cases it may be sufficient to use a single standard mixture, 

 thus: Shawf found that he could exalt the virulence of anthrax 

 bacilli by cultivating them upon blood-serum agar for fourteen 

 generations, after which they were three times as active as cultures 

 similarly transferred upon ordinary agar-agar. 



The increase of virulence under such conditions probably depends 

 upon the immunization of the bacteria to the body juices of the 

 animals, and this whole matter will be understood after the subject 

 " Immunity" has been considered. 



Number. The number of bacteria entering the infected animal 

 has a very important bearing upon infection. 



The entrance of a single micro-organism of any kind is scarcely 

 ever able to cause infection because of the uncertainty of its being 

 able to withstand the changed conditions to which it is subjected. 

 In most cases a considerable number of organisms is necessary in 

 order that some may survive. Park points out that when bacteria 

 are transplanted from culture to culture, under conditions supposed 

 to be favorable, many of them die. It seems not improbable, there- 

 fore, that when they are transplanted to an environment in which are 

 present certain mechanisms for defending the organism against them, 

 many more must inevitably die. The more virulent an organism 

 is, the fewer will be the number required to infect. Marmorek, 



* Directions for making and using the capsules are given in the chapter upon 

 Animal Experimentation. 



f "Brit. Med. Jour.," May 9, 1903. 

 6 



