RESPIRATION AND FERMENTATION 267 



obligate aerobic forms, which perish like most higher plants in the 

 absence of oxygen ; facultative anaerobes, able to live in the pres- 

 ence of air or without it, and obligate anaerobes, dying when 

 exposed to free oxygen. The butyric-acid bacteria belong to 

 the last group. 



Of the several types of butyric-acid bacteria the most charac- 

 teristic are those belonging to the genus Clostridium. They cause 

 milk and butter to turn rancid and are of economic importance 

 in the maturing of cheese. Here belongs also Clostridium pasteur- 

 ianum already considered, which fixes molecular nitrogen. The 

 genus Clostridium is characterized by the production of very resist- 

 ant spores, some of which will resist even boiling for a short time. 

 Hence butyric-acid fermentation may take place in products which 

 previously have been boiled. Insufficiently boiled milk, for 

 instance, is especially apt to become rancid, since the air has been 

 expelled by boiling. Moreover, the competing lactic-acid bac- 

 teria are removed by boiling, while in fresh milk they precede in 

 their development all other microorganisms. 



The material used for butyric-acid fermentation is sugar and 

 the chief products of it are butyric acid, carbon dioxide, and 

 hydrogen. It may be represented by the equation 



C0H12O0 = CH3CH2CH2COOH + 2C0 2 + 2H 2 . 



Still, it practically never proceeds according to this equation, 

 but produces a series of additional products, such as lactic and 

 acetic acids, ethyl alcohol, and methane (CH4). Many species of 

 butyric-acid bacteria produce very effective enzymes capable of 

 hydrolyzing and fermenting the most stable compounds, for 

 example, cellulose, which usually resists the action of other micro- 

 organisms. 



Bacteria of butyric fermentation are widely distributed in 

 nature. The soil is literally crowded with them. They play an 

 important role in the disintegration of the most stable plant resi- 

 dues. They develop at the bottom of bogs and their activity 

 explains the production of marsh gas, consisting of methane 

 and hydrogen, as well as phosphine, which causes spontaneous com- 

 bustion of the gas with the production of " Jack o' Lanterns." 



Acetic-acid fermentation differs somewhat from other kinds of 

 fermentation. Its chief difference lies in the fact that the material 

 used, ethyl alcohol, is not broken up, but is oxidized; hence it 



