36 BACTERIA. 



acetic acid (acetous-fermentation) and forms a greyish covering 

 of Bacteria (" Vinegar-mother ") on the surface of the liquid ; the 

 acetic acid formed, becomes by continued oxidization by B. aceti, 

 again transformed into carbonic acid and water. Aerobic; 

 short cylindrical cells, often united into chains, or to form a 

 zoogloea ; sometimes also rod- and spindle-shaped. The Vinegar- 

 bacteria and other kinds with ball- or rod-forms sometimes become 

 swollen, spindle-shaped, or oval links; they are supposed to be 

 diseased forms 1 ("Involution-forms"). 



Bacillus laciicus (Bacterium, acidi lactici, Zopf) is always found 

 in milk which has stood for some time, and in sour foods (cabbage, 

 cucumbers, etc.) ; it turns the milk sour by producing lactic acid 

 fermentation in the sugar contained in the milk ; the lactic acid 

 formed, eventually causes the coagulation of the casein. It re- 

 sembles the Vinegar-bacteria, occurring as small cylindrical cells, 

 rarely in short rows; not self-motile. Several other Bacteria appear 

 to .act in the same way, some occurring in the mouth of human! 

 beings ; some of these Bacteria give to butter its taste and flavour. 



The 'kefir-grains which are added to milk for the preparation of 

 kefir, contain in large numbers a Bacterium (Dispora caucasica) 

 in the zooglcea-form, a Yeast-fungus, and Bacillus lacticus. Kefir 

 is a somewhat alcoholic sour milk, rich in carbonic acid ; it is a 

 beverage manufactured by the inhabitants of the Caucasus, from 

 the milk of cows, goats, or sheep, and is sometimes used as a 

 medicine. In the production of kefir, lactic acid fermentation 

 takes place in one part of the sugar contained in the milk, and 

 alcoholic fermentation in another part, and the casein which had 

 become curdled is partially liquefied (peptonised) by an enzyme of 

 a Zooglcea-bacterium. 



Bacillus amylobacter (Bacillus butyricusj, the Butyric- acid-bac- 

 terium (Fig. 29), is a very common anaerobic which produces 

 fermentation in sugar and lactic-acid salts, and whose principal 

 product is butyric acid. It destroys articles of food and (together 

 with other species) plays a part in the butyric acid fermentation 

 which is necessary in the making of cheese ; it is very active 

 wherever portions of plants are decaying, in destroying the cellu- 

 lose in the cell-walls of herbaceous plants, and is thus useful in 

 the preparation of flax and hemp. The cells are self-motile, 

 generally cylindrical, sometimes united into short rows ; endo- 



1 According to Hansen these are not disease forms, but occur regularly under 

 certain conditions, e.g. temperature. 



