BY LIVING CULTURES. 465 



Exaltation of the Virulence. The converse process to 

 attenuation, i.e., the exaltation of the virulence, is obtained 

 chiefly by the method of cultivating the organism from 

 animal to animal the method of passage discovered by 

 Pasteur (first, we believe, in the case of an organism 

 obtained from the saliva in hydrophobia, though having no 

 causal relationship to that disease). This is most con- 

 veniently done by intraperitoneal injections, as there is less 

 risk of contamination. The organisms in the peritoneal 

 fluid may be used for the subsequent injection, or a culture 

 may be made between each inoculation. The virulence of 

 a great number of organisms can be increased in' this 

 way, the animals most frequently used being rabbits and 

 guinea-pigs. This method can be applied to the organisms 

 of typhoid, cholera, pneumonia, to streptococci, and staphylo- 

 cocci, and in fact to those organisms generally which invade 

 the tissues. 



The virulence of an organism, especially when in a 

 relatively attenuated condition, can also be raised by inject- 

 ing along with it a quantity of a culture of another organism 

 either in the living or dead condition. A few examples 

 may be mentioned. An attenuated diphtheria culture 

 may have its virulence raised by being injected into an 

 animal along with the streptococcus pyogenes ; an attenuated 

 culture of the bacillus of malignant oedema by being 

 injected with the bacillus prodigiosus ; an attenuated 

 streptococcus by being injected with the bacillus coli, etc. 

 A culture of the typhoid bacillus may be increased in 

 virulence, as already stated, by being injected along with a 

 dead culture of the bacillus coli. In such cases the ac- 

 companying injection enables the attenuated organism to 

 gain a foothold in the tissues, and it may be stated as a 

 general rule that the virulence of an organism for a particular 

 animal is raised by its growing in the tissues of that 

 animal. 



Combination of Methods. The above methods may be 

 combined in various ways. By repeated injections of 

 cultures at first attenuated and afterwards more virulent, 

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