THE CHEMISTRY OF BACTERIA. 99 



access of oxygen is necessary to this change, which is repre- 

 sented by this equation: 



0,HeO + O, = C,H,0, + H,0. 



Alcohol. Acetk acid. 



It was supposed by Liebig that this was a purely chem- 

 ical reaction, but the studies of Pasteur showed that living 

 organisms were necessary to effect the change. The liquid 

 undergoing this kind of fermentation becomes covered with 

 a slimy scum, which in reality is a zooglea, or mass of various 

 bacteria. Inasmuch as experience has taught that the con- 

 version of alcoholic liquids into vinegar was hastened by 

 the addition of some of this scum, the latter came to be 

 known as "mother of vinegar." It was subsequently 

 named mycoderma aceti. This term, however, must not be 

 understood to designate a single bacterial species. 



The production of acetic acid is not limited to any one 

 organism. From the mother of vinegar various kinds of 

 bacteria have been isolated. As yet, pure cultures have not 

 been employed for the preparation of vinegar. Among those 

 which have been studied may be mentioned the M. aceti, 

 Bacterium aceti, Bacillus aceticus, B. Pasteurianus, etc. 

 These organisms exhibit a marked tendency to give rise to 

 involution forms, especially, when grown at about 40° 

 (Fig. 3, p. 21). 



Acetic acid fermentation does not occur in liquids which 

 contain more than 15 per cent, of alcohol. Moreover, it is 

 not possible to obtain by direct fermentation a vinegar 

 which will contain more than about 14 per cent, of acetic 

 acid. A temperature of 30-35° is most favorable to the 

 change. When the nutrient material becomes exhausted 

 the bacteria present may attack the acetic acid which they 

 produced, and, may oxidize this to carbonic acid and water., 



CUfi, + 2 O, = 2 CO, -f 2 H,0. 



