24 PHYSIOLOGY OF BACTERIA 



pass through also. As a rule, we find that the cells 

 change insoluble compounds into a soluble form by 

 secreted enzymes. The energy liberated by secreted 

 enzymes usually is small, that from endo-enzymes is 

 quite large. (Table 5.) 



The chemical decomposition of compounds within 

 the bacterial cell, for the purpose of liberating energy, 

 shall be taken in this book as a definition of fermentation. 

 It is practically the same definition which was given by 

 Hugo Fischer (1902) : 



1. Fermentation is an intracellular process. 



2. It yields products essentially different from the 

 materials fermented and which are not mere parts of this 

 material. 



3. The end products are useless or harmful to the cell. 



4. It produces ''vital" energy (i.e., energy available 

 for cell processes). 



The definition includes all processes, organic and 

 inorganic, aerobic and anaerobic, with and without gas 

 formation, of proteins, fats, carbohydrates, acids, etc., 

 if only they fit the above definition. 



That fermentations provide cell energy as respiration does with 

 animals, was recognized by Pasteur (1876), though he did not furnish 

 absolute proof for this statement. He proved that yeast cells grew 

 in the absence of oxygen, and under these conditions caused fermenta- 

 tion, but he did not exclude other possible sources of energy, especially 

 that from the proteins. Since then, several cases have become 

 known where fermentation is the only source of energy, under condi- 

 tions where no other source is imaginable. To this group belongs 

 the fermentation of a mineral-ammonia-sugar solution by yeast, 

 of a mineral-urea solution, with traces of organic acids, by urea 

 bacteria, and of a mineral-ammonia solution by nitrate bacteria. 

 The growth of Oidium lactis and of Mycoderma in a lactic acid- 

 mineral solution -fNHs, and the growth of most prototrophic 

 organisms are examples of simple oxidations furnishing the energy 

 for growth. 



