58 MICRO-ORGANISMS AND FERMENTATION. 



extensive experiments, that a slight concussion is beneficial 

 to the life and propagation of micro-organisms ; at a certain 

 rate of vibration the propagation of the species is greatest; 

 if the concussion is increased, the propagation is checked in 

 proportion. But the optimum and maximum are different 

 in different species. 



PASTEUR made the important discovery that there are certain 

 bacteria and other micro-organisms which do not require free 

 oxygen, but are capable of producing very active decomposition 

 of the fermenting material even when oxygen is excluded. He, 

 therefore, distinguished two classes of micro-organisms, naming 

 the last-mentioned anaerobic and the others aerobic. More 

 recently DUCLAUX has stated that there are intermediate 

 forms between the two extremes. As an example of anaerobic 

 bacteria, PASTEUR'S bacterium of the butyric acid fermentation 

 may be mentioned. 



We will now pass in review the more important species 

 which are of special interest in the fermentation industries. 



1. ACETIC ACID BACTERIA. 



The acetic acid bacteria were first thoroughly examined 

 from a morphological and biological standpoint by HANSEN. 



As early as 1837-38 the view was expressed by TURPIN 

 and KUETZING that the acetic acid fermentation is caused 

 by a micro-organism, which KUETZING described and deline- 

 ated under the name of Ulvina aceti. Starting from this, 

 PASTEUR, first in his treatise (1864) and subsequently in 

 his work "Etudes sur le vinaigre " (1868), furnished experi- 

 mental proof of the correctness of this view, and also gave 

 a method, based on his results, for the manufacture of vinegar. 

 He assumed that the acetic acid fermentation was caused by a 

 single species of micro-organism which he called Mycoderma 

 aceti. Subsequent research has, however, shown that there are 

 different species of acetic acid bacteria. As far as PASTEUR'S 

 work was concerned, therefore, it was not at all a question of the 

 employment of one definite, selected species. His method consists 

 in giving a large surface to the liquid employed two parts of 

 bright wine to one part of wine- vinegar and then sowing on 



