130 PRODUCTS OF VITAL ACTIVITY. 



A considerable number of bacteria are able to produce lactic 

 acid from milk sugar and other carbohydrates. One of these is 

 considered the special lactic-acid ferment Bacillus acidi lactici and 

 is the usual cause of the acid fermentation of milk. Pure cultures 

 of this bacillus introduced into sterilized milk or solutions of milk 

 sugar, cane sugar, dextrin, or mannite, give rise to the lactic-acid 

 fermentation, in which carbonic acid is also set free. The process 

 requires free access of oxygen, and progresses most favorably at a 

 temperature of 35 to 40 C., ceasing at about 45. In milk, coagu- 

 lation of the casein occurs within fifteen to twenty-four hours after 

 adding a small quantity of a pure culture of the lactic-acid bacillus. 

 This is not due, however, to the acid fermentation, but to a ferment 

 resembling that of rennet, which is produced by many different 

 bacteria, some of which do not produce an acid reaction of the milk. 

 Among the bacteria which produce lactic acid from milk sugar we 

 may mention the staphylococci of pus, Bacillus lactis aerogenes, and 

 Bacillus coli communis. 



The formula showing the transformation of sugar into lactic 

 acid is usually stated as follows : CjH^O,, = 2(HC 3 H 6 O 3 ). 



Acetic acid is also produced from dilute solutions of alcohol by 

 the action of a special bacterial ferment, which accumulates upon 

 the surface of the fluid as a mycoderma, consisting almost entirely 

 of the Bacillus aceticus (Mycoderma aceti). Free access of oxygen 

 is required, and a temperature of about 33 C. is most favorable to 

 the process. According to Duclaux, the " Mycoderma aceti " oxi- 

 dizes the alcohol, in solutions containing it, so long as any is present, 

 and when it is exhausted it oxidizes the acetic acid previous^ 

 formed by oxidation of the alcohol, producing from it carbon diox- 

 ide and water. 



The formation of acetic acid from alcohol is shown by the follow- 

 ing formula : Ethyl alcohol CH S .CH 9 .OH + O 8 = CH 3 .COOH + H,O. 



Butyric acid is produced by a considerable number of bacteria, 

 one of which, named Bacillus butyricus, has received the special at- 

 tention of Prazmowski. This is strictly anaerobic. In solutions of 

 starch, dextrin, sugar, or salts of lactic acid, when oxygen is ex- 

 cluded it produces butyric acid in considerable quantity, and at the 

 same time carbon dioxide and hydrogen gas are set free. Duclaux 

 gives the following formula of a solution containing lactate of lime 

 in which the action of the butyric-acid ferment may be well studied : 



Water, . . . . . . 8 to 10 litres. 



Lactate of lime (pure), . . . .. 225 grammes. 



Phosphate of ammonia, . . . .0.75 " 



Phosphate of potash, . . . . .. 0.4 



Sulphate of magnesa, . . . .0.4 



Sulphate of ammonia, . . . . 0.2 " 



