122 BACTERIA AND THE CARBON DIOXIDE CYCLE 



In some cases, of course, the fermentation must be preceded by inversion 

 by enzymes. The other two species mentioned are, as their names imply, 

 more particular, and need grape, cane, or milk sugar as food. 



2-4 grams of glucose fermented by B. orthobutylicns yielded in twenty 

 days 



Grams. 



0-842 Normal Butyric acid 



0-264 Butylic alcohol 



0-229 Acetic acid 



besides free hydrogen and C(X. These gases increased by continued fer- 

 mentation, probably showing that the organism can split up its own pro- 

 ducts into still simpler compounds down to CO 2 . 



A granulose-bearing species from the soil, described by Beyerinck as 

 Granulobacter butyricus, is particularly worth notice, since it forms from 

 maltose, not butyric acid, but butylic alcohol with some CO 2 and free H. 



Not infrequently butyric acid arises as a product in putrefactive pro- 

 cesses, and it seems that some butyric bacteria (B. butyricns, Hueppe) have 

 true saprogenic properties, and can break up proteids. Other species, like 

 B. orthobutylicus, can form butyric acid from peptone only in the presence 

 of one of the above-mentioned non-nitrogenous bodies. 



Milk soured by B. lactis is often infected by butyric organisms 

 which disintegrate both the remaining milk sugar and the lactic acid that 

 has arisen from it. The fermentation of the calcium salt of lactic acid ex- 

 hibits this change in an uncomplicated form *. 



Cellulose Fermentation. 



In manure heaps and in the rotting vegetation at the bottom of lakes 

 and ditches there is going on side by side with the disintegration of 

 carbohydrates in plant tissues another process, a special fermentation of 

 cellulose known as methane or marsh gas fermentation (95). The organ- 

 isms which effect this change first of all ' invert' the cellulose (i. e. break it 

 up into a sugar) by means of an enzyme 



and then ferment it to methane CH 4 and carbon dioxide, fatty acids arising 



as by-products. Methane, mixed with free hydrogen and CO 2 , rises to 



the surface when the mud at the bottom of ponds is stirred up with a stick. 



There seem to be a considerable number of methane bacteria. The 



Vibrio rugula (anaerobic, granulose-bearing) probably is one. Another 



species (a delicate anaerobic motile Plectridiuni) that has been isolated from 



* For butyric bacteria in cheese-making see p. 119; their distribution in nature, p. 135 ; their 

 assimilation of nitrogen in the soil, p. 96. 



