460 IMMUNITY 



coccus. Pasteur found in the case of chicken cholera, that 

 when cultures were kept for a time in ordinary conditions, they 

 gradually lost their virulence, and that when sub-cultures were 

 made the diminished virulence persisted. Such attenuated 

 cultures could be used for protective inoculation. He considered 

 the loss of virulence to be due to the action of the oxygen of 

 the air, as he found that in tubes sealed in the absence of oxygen 

 the virulence was not lost. Haffkine attenuated cultures of the 

 cholera spirillum by growing them in a current of air (p. 412). 



(2) The virulence of an organism for a particular animal may 

 be lessened by passing the organism through the body of another 

 animal. Duguid and Burdon Sanderson found that the virulence 

 of the anthrax bacillus for bovine animals was lessened by its being 

 passed through guinea-pigs, the disease produced in the ox by 

 inoculation from the guinea-pig being a non-fatal one. This dis- 

 covery was confirmed by Greenfield, who showed that the bacilli 

 cultivated from guinea-pigs preserved their property in cultures, 

 and could therefore be used for protective inoculation of cattle. 

 A similar principle was applied in the case of swine plague by 

 Pasteur, who found that if the organism producing this disease 

 was inoculated from rabbit to rabbit, its virulence was increased 

 for rabbits but was diminished for pigs. The method of vaccina- 

 tion against smallpox depends upon the same principle. There 

 is also evidence to show that the virulence of the tubercle 

 bacillus becomes modified according to its host, being often 

 diminished for other animals. 



(3) Many organisms become diminished in virulence when 

 grown at an abnormally high temperature. The method of 

 Pasteur, already described (p. 314), for producing immunity in 

 sheep against anthrax bacilli, depends upon this fact. A virulent 

 organism may also be attenuated by being exposed to an elevated 

 temperature which is insufficient to kill it, as was found by 

 Toussaint in the case of anthrax. 



(4) Still another method may be mentioned, namely, the 

 attenuation of the virulence by growing the organism in the 

 presence of weak antiseptics. Chamberland and Koux, for 

 example, succeeded in attenuating the anthrax bacillus by 

 growing it in a medium containing carbolic acid in the 

 proportion of 1 : 600. 



These examples will serve to show the principles underlying 

 attenuation of the virulence of an organism. There are, how- 

 ever, still other methods, most of which consist in growing the 

 organism in conditions somewhat unfavourable to its growth, e.g. 

 under compressed air, etc. 



