306 PASTEUR I THE HISTORY OF A MIND 



exclusively chemical, a toxic role, demonstrated by 

 P. Bert. All microbes require a small amount of oxygen 

 and are injured by an excess of it. The anaerobes must 

 have traces of it but die in ordinary air. The aerobes 

 live in ordinary air but die in compressed oxygen. Be- 

 tween the physiological limits and the toxic limits there 

 is, moreover, a zone of attenuation, studied by M. 

 Chauveau for the anthrax bacteridium. 



After oxygen, come naturally the antiseptics which, 

 likewise, when present in very small proportions, are 

 harmless or even beneficial to the microbes, but if 

 present in larger quantities kill them. MM. Chamber- 

 land and Roux have studied the action of phenic acid, 

 of bichromate of potash and of sulphuric acid on the 

 anthrax bacteridium and have discovered in this way 

 some curious facts to which we shall soon return. 



In short, there are several means of producing from the 

 same virulent race a whole series of races more and more 

 attenuated. Up to this time we have studied them only 

 as vaccines. In order fully to investigate their role from 

 this new point of view, it is necessary to study them in 

 themselves. 



How do bacteridia which are not equally attenuated 

 differ physiologically? They are very much alike in 

 bouillon cultures. When attenuated they produce rods 

 which separate easily and diffuse through the culture 

 medium, clouding it, while the virulent bacteridia 

 adhere in flakes, which float in the midst of a clear liquid. 

 But their physiological needs are the same, and it is 

 almost impossible to differentiate them by means of the 

 microscope. For that purpose they must be inoculated 

 into living creatures. 



Let us study them in animals. In proportion as the 

 bacteridium becomes attenuated, we find that it ceases, 

 first, to become virulent for cattle, but that it is still 



