BY LIVING CULTURES 461 



(b) Immunity by living Virulent Cultures in Non-lethal 

 Doses. Immunity may also be produced by employing virulent 

 cultures in small, that is non-lethal, doses. In subsequent 

 inoculations the doses may be increased in amount. For example, 

 immunity may thus be obtained in rabbits against the bacillus 

 pyocyaneus. Such a method, however, has had a limited 

 application in the case of virulent organisms, as it has been 

 found more convenient to commence the process by attenuated 

 cultures, and then to continue with living cultures. 



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 conveniently 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 staphylococci, 

 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 injecting 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 accompanying in- 

 jection 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 com- 

 bined in various ways. By repeated injections of cultures at 

 first in the dead condition, then living and attenuated and 



