56 MICRO-ORGANISMS AND FERMENTATION. 



by continued investigation endogenous spores may also be 

 found in all species of tbe last-mentioned group. It is only 

 a supposition that the above-mentioned separated members 

 must be considered as analogous to the spores. 



Finally, in the morphology of bacteria, we must mention 

 the so-called zooglcea formation (Fig. 10 d). It is known that 

 in all branches of the fermentation industries, in places where 

 the cleaning is not strictly attended to, slimy, fatty masses 

 may occur, which gradually increase in thickness. The cause 

 of this is commonly a growth of bacteria occurring in such a 

 manner that the single cells lie very close to each other, whilst 

 at the same time the outer gelatinous layers of the cell-wall 

 greatly swell up. During the continued growth of the 

 bacteria the slimy layer increases in thickness, and often 

 assumes certain characteristic forms. Such slimy masses 

 known in the sugar manufacture as "frog-spawn" occur 

 both on solid and in liquid media. 



Pasteur made the important discovery, that there are 

 certain bacteria and other micro-organisms which do not 

 require free oxygen, and even produce very active decomposi- 

 tions of the fermenting material 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 fermenta- 

 tion 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 w^ere first thoroughly described 

 from a morphological standpoint by Hansen. The correct- 

 ness of his investigations was afterwards confirmed by Zopf, 

 deBary, and A. J. Brown. 



As early as in the year 1838 the view was expressed by 



