76 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 elimin- 

 ating those that cannot endure the defensive agencies of 

 those juices with which the organisms come in contact. 



3. 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. In 

 adding blood-serum, ascitic fluid, pleuritic exudate, etc., to 

 bouillon, agar-agar, and other standard media, the body 

 juices become too dilute for the same action to take place as 

 in the "passage" or collodion sac, and this dilution of the 

 constituents probably explains the slowness of the method 

 in cases in which it succeeds, and the failures in cases in which 

 it does not. Shaw* 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 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. 



2. Number. The number of bacteria entering the in- 

 fected animal has a very important bearing upon infection, 

 and may determine whether it shall occur or not. 



It seems to be true of very few varieties of bacteria that 

 the entrance of a single one may produce infection. In 

 most cases a number of organisms are necessary in order 

 that some may survive the sudden introduction to the 

 new environment. Park points out that when bacteria are 

 transplanted from culture to culture, 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. Mar- 

 morek, in his experiments with antistreptococcic serum, 

 used a streptococcus whose virulence was exalted by passage 

 through rabbits and intermediate cultivation upon agar- 

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



