THE PASTEUR EFFECT 319 



Methods for the study of such nutritional requirements as are of 

 use in taxonomy are given by SteUing-Decker and have been briefly 

 treated in Chapter III. 



THE PASTEUR EFFECT 



That aerobic conditions inhibit the anaerobic breakdown of hexoses 

 and other carbohj^drates was noted by Pasteur in his Etudes sur la 

 BiereP He observed that the fermentation * carried on by cells of 

 facultative organisms under anaerobic conditions disappeared or 

 diminished and was replaced by respiration when such cells were 

 placed under aerobic conditions, e.g., fermentation carried on by 

 yeast cells (as indicated by the production of ethyl alcohol) under 

 anaerobic conditions diminished (as indicated by the diminution of 

 the production of alcohol) on aeration of the culture, and was re- 

 placed by respiration (as indicated by the increased amount of carbon 

 dioxide formed). This phenomenon of the inhibition of fermentive 

 processes by oxygen is the so-called Pasteur effect or Pasteur phe- 

 nomenon. 



One might regard this as a manifestation of a regulatory device 

 whereby facultative organisms can use either their aerobic or an- 

 aerobic systems to obtain energy from the breakdown of sugar. Such 

 a device is advantageous to the organism. In the presence of suffi- 

 cient oxygen, the fermentive mechanism is blocked off and energy 

 is obtained by the more efficient respiratory mechanism. Only in 

 the absence of sufficient oxygen is the less efficient fermentive 

 mechanism brought into play. Less sugar is required to develop the 

 population of facultative organisms to a certain point under aerobic 

 conditions than under anaerobic. The latter statement should not 

 be misconstrued to mean that sugar consumption is lowered under 

 aerobic conditions. For a given amount of sugar, provided there 

 is an adequate supply of other nutrients, the population of faculta- 

 tive organisms in a culture will attain a higher point under aerobic 

 conditions than under anaerobic conditions in a given period of time. 



Many theories have been advanced concerning the mechanism and 

 the exact locus of the Pasteur effect. None, however, has been gen- 

 erally accepted. Some claim that glucose is broken down to inter- 



* The term fermentation will be used in this discussion of the Pasteur effect 

 to refer to those processes whereby organisms obtain energy for their use by 

 the anaerobic dissimilation of carbohydrates, i.e., anaerobic glycolytic processes, 

 and the term respiration to those processes whereby organisms obtain energy 

 by the aerobic breakdown of carbohydrates. 



