304 PASTEUR I THE HISTORY OF A MIND 



VII 

 VIRULENCE AND ATTENUATION 



Attenuation is a general phenomenon. After having 

 determined its occurrence in chicken cholera, the 

 anthrax bacteridium, rabies, and the organism of 

 erysipelas of the pig, 1 Pasteur found it in a microbe occur- 

 ring in horses which had died from typhoid fever, and 

 in another organism derived from the saliva of a child 

 attacked by hydrophobia, which last mentioned or- 

 ganism was found later to be the pneumococcus of 

 Talamon-Fraenkel. All these bacilli became attenuated 

 when they were allowed to grow old in the fluid culture 

 medium. 



But what do we mean by this expression "grow old"? 

 Age is a result, and cannot be an active cause. It ac- 

 companies attenuation, it does not produce it; or, rather, 

 the same cause which produces one, at the same time 

 produces the other. When we search for some physico- 

 chemical influence which might come into play, we think 

 at once of oxygen. 



The micrococcus of chicken cholera is, for example, 

 an ae'robe in the culture flask and in the organism. 

 When it ceases to multiply in the culture medium, it 

 continues to respire there, to give off carbonic acid 

 by consuming its own tissues. It contracts and shrinks 

 visibly (Fig. 22). Its attenuation, which is a proof of 

 its debility, is probably due to this internal process, and 

 in reality experiment teaches that when the supply of 

 oxygen is limited by sealing the tubes, allowing only a 

 small amount of this gas to be present, the virulence is 

 maintained much longer. It is the same in all cases, 

 and always the oxygen, regarded as the agent of com- 



1 Fr. Rouget de pore; Ger. Schweinerothlauf. Trs. 



