CHAPTER XLIV. 



ACETIC-ACID BACTERIA. 



NITRIFICATION consists in an oxidation of nitrogen, hence the 

 nitrifying bacteria are oxidizing microorganisms. Bacterial oxida- 

 tions are not confined to changes in the soil which are of the greatest 

 significance to agriculture, but they play an important role in nature in 

 the cycle of certain elements and in certain industries. One of these is 

 the essential chemical process in the manufacture of vinegar, consist- 

 ing in the conversion of alcohol into acetic acid. Alcohol formation 

 from sugar is generally due to yeast cells (blastomycetes or budding 

 fungi), and the conversion of liquids, such as beer, wine, or cider, 

 containing alcohol into vinegar, that is, into a fluid containing acetic 

 acid, is due to bacterial microorganisms. When alcohol containing 

 liquids in open vessels and exposed to the air are changed into 

 vinegar a tenacious slimy membrane is formed on the surface. It 

 was noted long ago that pieces of this membrane rapidly changed 

 alcoholic liquids into vinegar in the presence of free air; the membrane, 

 therefore, received the name mother of vinegar (mere du vinaigre, 

 French; Essigmiitter, German). Persoon, in 1822, examined such 

 membranes in various fermentative processes, and he called them 

 mycoderma, which means a fungus, or slimy skin or membrane, but 

 he did not believe in any causal connection between them and the 

 fermentative process. Kuetzing, a German botanist, however, in 1832, 

 declared that the mother of vinegar consisted of exceedingly small, 

 punctate algae, and that the conversion of alcohol into acetic acid was 

 due to their metabolism. Kuetzing's claims never attracted much 

 attention, and had long been forgotten, when more than forty years 

 later Pasteur, in his studies on fermentations, again maintained that 

 the conversion of alcohol into acetic acid was due to the life activity 

 of microorganisms which he called Mycoderma aceti. Pasteur, how- 

 ever, did not exhaustively study the morphology of these vinegar 

 organisms; this was later done by Hansen, who distinguished three 

 different species of bacteria as the cause of the acetic-acid formation 

 from alcohol. These he named Bacterium aceti, Bacterium Pasteuri- 

 anum, and Bacterium Kuetzingianum. The three species are found 

 in beer which is not very rich in alcohol and which is undergoing 

 acetic-acid fermentation. Bacterium aceti forms a moist, slimy, 

 smooth pellicle with fine lines; Bacterium Pasteurianum, a rather 

 dry pellicle; and Bacterium Kuetzingianum one which resembles that 

 of Bacterium aceti, but is much thicker, elevated, and reaches up along 



