92 MICRO-ORGAN ISMS AND FERMENTATION. 



As soon, however, as a place has been found in the system 

 for any pure species, it is necessary to distinguish it by a 

 detailed and thorough investigation, and to give such an 

 exact description that it can be clearly defined in relation to 

 other species, and may be recognised again if new pure cultures 

 were prepared from the impure material. After the far- 

 reaching work of the last few years, and the thorough explor- 

 ation to which the field has been subjected, it is possible to 

 give such a description of many species of bacteria, notwith- 

 standing the difficulties due to the great tendency of these 

 plants to exhibit variations. In such a description of a species, 

 it is necessary, owing to the poverty of these micro-organisms 

 in definite shape, to take into consideration many other points. 

 Cohn established the classification of bacteria already men- 

 tioned by means of their physiological properties, dividing 

 them into pathogenic, zymogenic, and ehromogenic bacteria. 

 Of these, the second group is of importance to us, and the 

 following examples embody typical species of those bacteria 

 that are distinguished either by their special enzymes and 

 fermentative products acetic acid, lactic acid, etc. or by 

 their action upon fermenting liquids. 



1. Acetic Acid Bacteria. 



The acetic fermentation is a process of oxidation. By the 

 activity of the bacteria in presence of oxygen, the alcohol in 

 the liquor is oxidised to aldehyde and water, and then the 

 aldehyde is further oxidised to form acetic acid. It is a 

 process differing greatly from those classed as fermentations, 

 and has a certain similarity with the process of breathing. 

 The entire content of alcohol undergoes change without the 

 production of by-products ; the bacteria can, however, bring 

 about the combustion of acetic acid to form carbon dioxide 

 and water. It is, therefore, of importance to interrupt the 

 process as soon as acetic acid is formed, if the full yield is to 

 be obtained. 



Persoon, as early as 1822, was acquainted with the vegetable 

 character of the film which forms on the surface of liquids under- 

 going acetic fermentation, and he named the film Mycoderma. 



