260 APPLIED BACTEEIOLOGY 



this view, and also published a method based on the 

 results, for the manufacture of vinegar. He assumed, 

 however, that the acetic fermentation was caused by one 

 species of organism, which he called the Mycoderma aceti. 

 Subsequent research, however, has shown that there are 

 different species of acetic acid bacteria. 



The fermentations which are known to be due to 

 bacterial life can be conveniently classed under four head- 

 ings as follows, according to the chemical change they 

 induce in the fermentable substances : 



1. Fermentation by oxidation. 



2. Fermentation by hydration. 



3. Fermentation by simple decomposition. 



4. Fermentation by reduction. 



We will now consider a few of the more important fer- 

 mentations which fall under these respective headings. 



1. Fermentation by Oxidation. — There are two very im- 

 portant fermentations belonging to this group — the acetic 

 fermentation of alcohol, and the oxidation of ammonia into 

 nitrates, which takes place in the soil. 



Acetic Fermentation of Alcohol. — The conversion of wine 

 and other alcoholic liquids into vinegar, on prolonged ex- 

 posure to the air, is a phenomenon which has been known 

 from the earliest times. As has already been stated, 

 Pasteur first showed that the cause of this oxidation was 

 the Mycoderma aceti. Hansen found, however, that this 

 organism consisted of two species, which he named the 

 Bad. aceti and Bact. Pasteurianum respectively. The Bact. 

 aceti consists of short bacilli about 2 /^ long, slightly con- 

 tracted in the middle, so that they somewhat resemble the 

 figure 8, and occur in chains of various lengths. Abnormal 

 forms are frequently seen, particularly in old cultures, 

 which frequently attain a length of 10 to 15 /u, or more, 

 and are often swollen into irregular shapes. The free 



