160 ATLAS OF BACTERIOLOGY. 



(d) By repeated inoculation of unsuitable animals. 

 The bacilli of hog erysipelas become much less viru- 

 lent from passing repeatedly through the rabbit; the 

 organisms of variola (these are not bacteria) from 

 passing through the body of the cow. 



It is much more difficult to increase the virulence 

 of those bacteria which have been attenuated. On 

 the whole, it may be said that the virulence returns 

 spontaneously so much more readily the more rapidly 

 the attenuation has been effected. 



Varieties which have slowly and spontaneously lost 

 their virulence may often be restored to increased 

 virulence in the following ways : 



1. Culture in bouillon to which ascites fluid has 

 been added (streptococci, diphtheria). 



2. We first infect especially sensitive animals par- 

 ticularly very young ones, such as young guinea-pigs 

 and, when these have succumbed, convey the germs 

 (directly with the blood of the animals) to older and 

 more resistant animals of the most sensitive species, 

 later to more resistant species. Each passage through 

 an animal increases the virulence until finally a certain 

 maximum is reached. 



3. Sensitive animals are infected with large amounts 

 of the fresh bouillon culture of the bacteria. The 

 metabolic products, which are introduced at the same 

 time, then increase the predisposition for the injected 

 organism. 



4. Large amounts of the metabolic products of bac- 

 terium vulgare are injected with the bacteria (this has 

 been especially useful in the case of staphylococci 

 and streptococci). The explanation of the effect is 

 the same as that of 3. 



