96 INFECTION 



when transfers to fresh media are made at prolonged intervals. This 

 decrease probably depends upon an actual decrease in virulence, and 

 particularly upon the selection, in artificial growth, of the less virulent 

 or vegetative forms which grow actively and soon exceed in number 

 their more pathogenic fellows. Each time the culture is transplanted 

 more of the vegetative and fewer of the pathogenic microorganisms 

 are carried over, until finally the pathogenic bacteria are entirely elim- 

 inated, or their virulence totally destroyed, and the entire culture is 

 composed only of vegetative or harmless forms of bacteria. 



Various other agencies lead to artificial lessening of virulence, such 

 as exposure, for short periods of time, to a temperature just under the 

 thermal death-point; exposure to sunlight; exposure to small quantities 

 of antiseptic or germicidal substances; the action of desiccation; sub- 

 jection to increased atmospheric pressure, etc., these methods being com- 

 monly employed in the preparations of vaccines to be used for purposes 

 of active immunization. 



The passage of a microorganism or virus through animals usually 

 increases its virulence, but may modify or attenuate it, as in the case of 

 the passage of smallpox virus through the calf, when it loses forever its 

 power of producing smallpox. 



Increase in virulence can best be secured by passing the microorgan- 

 ism through animals. It is practically impossible, by any means, to 

 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 



