160 PHYSIOLOGY OF BACTERIA 



''less digestible," though this term is rather meaningless 

 with bacteria acting upon dissolved food. 



The so-called selection of foods is probably linked 

 closely with the molecular structure of the cell, and a 

 comprehensive quantitative study of food selection 

 by bacteria might give us a clearer conception of the 

 cell mechanism. 



(e) SUMMARY OF FACTS 



The type of fermentation produced by a certain species 

 of bacteria depends above all upon the substrate. Similar 

 substrates, e.g., different sugars, may give the same 

 products with one species, and may give entirely different 

 products with another species. 



The same substrate may be decomposed by the same 

 organism in different ways, which depend upon the 

 environment. Such differences in type of fermentation 

 may be brought about by changes in acidity. Yeast in 

 alkaline solution forms large amounts of glycerol. 

 Clostr. aceto-ethylicum produces more volatile acids, more 

 alcohol and less acetone if the medium becomes alkaline. 



Temperature changes cause variations in the by-prod- 

 ucts, but not in the main type of fermentation. 



Differences in the concentration of the substrate may 

 bring about a different type of fermentation, as in the 

 citric acid fermentation by Aspergillus in sugar con- 

 centrations above 10%. 



SUMMARY OF THEORIES 



The basis for many of the possible different types of 

 fermentation has already been given extensively in the 

 chapter on Equations of Fermentation. 



Why one type changes into another, can not usually 

 be explained. It is understood only in the case where 



