VIRULENCE 97 



make a known non-virulent microorganism virulent, although it is 

 comparatively easy to increase the virulence of a culture that has be- 

 come well-nigh non-virulent on account of prolonged artificial cultiva- 

 tion. This fact is worthy of emphasis, and is well illustrated by the 

 large amount of work that has been done in fruitless attempts to render 

 non-virulent, diphtheria-like bacilli virulent by passage through various 

 animals or growth on special culture-media. 



In cases where the virulence is slight or absent, experimental manip- 

 ulations of the culture are directed toward gradual immunization of the 

 microorganisms to the defensive mechanism of the body of the animal 

 for which the organism is to be made virulent. This is well explained 

 according to the hypothesis of Welch, and will be referred to again in 

 the latter part of this chapter. A number of methods are made use of 

 for this purpose: 



(a) Passage through animals, which enables the microorganisms 

 gradually to immunize themselves or adopt certain morphologic and 

 biologic changes enabling them best to resist the defensive forces of the 

 host. Since these defensive forces vary with different animals, and 

 indeed with the various organs of the same animals, it is usual to find 

 that virulence raised by animal passage affects only the animal or the 

 particular organ of a certain animal, and not all animals in general. 

 Thus, in general, the passage of bacteria through rabbits increases their 

 virulence for rabbits and not for mice, dogs, pigeons, etc.; passage 

 through mice may increase their virulence for mice, but not for rabbits, 

 guinea-pigs, etc. 



(b) The use of collodion sacs for increasing virulence has been ad- 

 vocated, especially by French investigators. When microorganisms 

 are inclosed in a collodion capsule of the proper thickness and placed 

 within the abdominal cavity of a suitable animal, the slightly modified 

 body-juices are able to transfuse through the sac, impeding the develop- 

 ment of such microorganisms as are unable to immunize themselves or 

 withstand the injurious influences. In this manner a race of virulent 

 bacteria are artificially selected which can endure the defensive agencies 

 of those juices with which they have come into contact. 



(c) The addition of animal fluids to the culture-medium may enable 

 the bacteriologist to maintain or even to increase the virulence of a 

 microorganism according to the principles of artificial selection. The 

 fluid, either a serum or whole blood, is secured in a sterile manner and 

 added to the medium in a raw or unheated condition. In this manner 

 the microorganisms are exposed to some of the defensive agencies con- 



