78 Infection 



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 elimi- 

 nating those that cannot endure the defensive agencies of 

 those juices with which the organisms come in contact.* 



The addition of animal fluids to the culture-media some- 

 times enables the investigator to increase, and usually 

 enables him to maintain, the virulence of bacteria. The 

 cultivation of the organism should embrace a series of genera- 

 tions in gradually increasing concentrations of the body 

 fluid employed, until the organism becomes thoroughly 

 accustomed to it. 



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

 mixture, thus : Shaw f found that he could exalt the viru- 

 lence 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 under such conditions as these probably 

 depends upon the immunization of the bacteria to the body 

 juices of the animals, and this whole matter will be under- 

 stood after the subject " Immunity" has been considered. 



Number. The number of bacteria entering the infected 

 animal has a very important bearing upon infection, and 

 may itself determine whether it shall occur or not. 



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

 scarcely ever able to effect infection because of the uncer- 

 tainty of its being able to withstand the defensive mechanisms 

 of the animal into which it is introduced. In most cases 

 a considerable number of organisms is necessary in order 

 that some may survive the new environment. 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, therefore, 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. 



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

 chapter upon Animal Experimentation, 

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



