METABOLISM OF MICROORGANISMS 377 



2. The heterolactic fermentation {e.g., by L. pentoaceticus) turns about 

 half of the gkicose into lactic acid and converts the other half mainly 

 into carbon dioxide and ethyl alcohol. The equation is 



C6Hi206-» CH3 • CHOH • COOH + COo + CH3 • CHoOH 



Sometimes considerable amounts of acetic acid and small quantities of 

 glycerol are formed. 



These two types of lactic acid fermentation, homolactic and hetero- 

 lactic, are important in the industrial production of lactic acid and in 

 the making of cheese, sauerkraut, pickles, and silage. 



3. The propionic fermentation {e.g., by P. pentosaceum) gives propionic 

 acid, acetic acid, succinic acid, and carbon dioxide as major products, 

 but under certain conditions considerable amounts of lactic acid are 

 formed. The propionic fermentation may be regarded as superimposed 

 upon a homolactic fermentation, but it does not seem probable that lactic 

 acid is an intermediate in the production of propionic acid. In this and 

 the following fermentations the reactions are too complicated to be readily 

 expressed by simple equations. 



4. The colon-aerogenes-typhoid bacteria, not only produce all of the 

 compounds formed by the mixed lactics except glycerol, but in addition 

 make formic acid, hydrogen, and butylene glycol. This is a very hetero- 

 geneous group of organisms, and the proportion of the products to one 

 another varies greatly with the species of bacteria. Perhaps the most 

 distinctive products are: formic acid by Eberthella typhi, acids and 

 hydrogen by Escherichia coli, and acetoin and butylene glycol by Aero- 

 bacter aerogenes. 



5. The butyric acid fermentation {e.g., by CI. acetobutylicum) is char- 

 acterized by the almost complete absence of lactic acid and the appear- 

 ance of acetic acid, butyric acid, carbon dioxide, hydrogen, butyl alcohol, 

 ethyl alcohol, and acetone. Isopropyl alcohol may replace acetone 

 wholly or in part in certain butyric fermentations. Some bacteria in 

 this group do not form the last four compounds, collectively called 

 solvents, while others produce them in large amounts. 



6. The naturally occurring methane fermentation {e.g., by Methano- 

 bacterium omelianskii) involves a unique type of metabolism. The two 

 extremes of oxidized and reduced carbon products, carbon dioxide and 

 methane, meet here. This apparent contradiction did not appear so 

 strange when it was discovered that methane arose, at least partially, 

 by reduction of carbon dioxide with hydrogen. Methane (also called 

 marsh gas) occurs extensively in coal mines, stagnant waters, sewage, 

 certain types of plants, and in the intestinal tract of animals. 



7. Fairly well-characterized polysaccharides have been obtained from 

 more than 60 species of bacteria. Some of these give unusual products 



