116 INFECTION AND IMMUNITY. 



This insusceptibility of the tubercle bacillus and 

 the cocci mentioned to the germicidal action of 

 the serum is a natural and constant property. On 

 the other hand, certain organisms which are nat- 

 urally susceptible to the germicidal power of the 

 serum and leucocytes appear to acquire a resistance 

 against these agencies during the course of infec- 

 tion, and it is not at all unlikely that this is a 

 property which is common to all pathogenic micro- 

 organisms. We may look on this change as a 

 phenomenon of adaptation. In a distinct sense it 

 appears that the micro-organisms may become 

 immunized to the bactericidal substances of the 

 serum, and to the opsonins on which destruction 

 by phagocytosis depends. 



The increase of virulence in a micro-organism 

 by repeated passage through a suitable animal may 

 be regarded as an adaptation on the part of the 

 organism for the tissues and fluids of the animal, 

 and in this adaptation an increased resistance to 

 the germicidal substances probably is an important 

 factor. Similar agencies doubtless are at work in 

 the maintenance of virulence by growing cultures 

 on media which contain the serum of animals. 



Typhoid bacilli which have been cultivated from 

 the body of a patient recently are more resistant 

 to agglutination than those which have been on 

 artificial media for some time. The same has been 

 found true of the cholera vibrio (Pfeiffer and 

 Kolle), as well as of some other organisms. Also 

 the cultivation of bacteria (typhoid bacilli) in 

 media which contain an agglutinating serum brings 

 about an increased resistance to agglutination 

 (Eansom and Kitashima, Walker, Steinhardt and 

 others) ; and it has been shown that such modified 



