80 MICROBES AND TOXINS 



merely stereochemical difference in their composition." In 

 two culture media, identical except that one contains sorbite, 

 the other dulcite, a different sugar, the former supports the 

 sorbose bacterium, the latter is refractory. To the bacillus 

 tuberculosis distributed everywhere, all the " soils " are not the 

 same. The chemical study of the soil ought to go hand in 

 hand with the study of the microbe. 



Respiration : Anaerobic Life Oxygen is the primary 

 food of creatures which have respiration. Lavoisier showed 

 that oxygen is indispensable to life. 



During his study of the fermentative change of calcium 

 lactate into butyrate, Pasteur discovered the vibrio butyricus, 

 and made the fundamental observation that this organism lives 

 without free oxygen and even dies on contact with the air : 



" Pure carbonic acid passed for however long a period through 

 the fluid in which they are growing has no effect whatever on 

 their life and multiplication. If atmospheric air is passed 

 through instead for one or two hours under precisely parallel 

 conditions they all die, and the butyric fermentation which 

 depends on their presence ceases immediately." (Pasteur.) 



Those micro-organisms for which free oxygen seemed to be 

 a poison were called by Pasteur anaerobes. There exist " strict 

 anaerobes," " strict aerobes," which cannot exist without free 

 oxygen, and " facultative " bacteria capable of living in either 

 condition. 



In broth the aerobes form at the surface a little collar, a 

 ring, or a pellicle ; in a drop of fluid under the microscope 

 they can be seen to make their way towards the periphery, 

 where there is the best provision of oxygen. 



If a filament from a green alga, a plant containing chloro- 

 phyll, is put into a suspension of motile aerobes, and a small 

 spectrum of sunlight is allowed to fall upon it, the bacteria can 

 be seen collecting at the points where the chlorophyll assimila- 

 tion and the production of oxygen are most intense, i.e., at the 

 red and violet regions of the spectrum, the B and C lines and 

 the ^line of Frauenhofer (Engelmann's experiment). 



On the contrary, the anaerobes avoid the surface of the 



