PRODUCTS OF VITAL ACTIVITY. 137 



a small quantity (one ose) of the microorganism to be tested is intro- 

 duced. The culture medium is then quickly solidified by placing the 

 test tube in iced water. It is then placed in the incubator, and when 

 colonies form bubbles of gas will be seen in their vicinity, if the bac- 

 terium under observation is able to cause fermentation of glucose. 

 For accurate observations as to the quantity and nature of the gases 

 produced the fermentation tube should be used, as recommended by 

 Theobald Smith (see Fig. 38). 



Production of Acids. Numerous bacteria give an acid reaction 

 to the media in which they are cultivated, and the acids produced 

 are various lactic, acetic, butyric, propionic, succinic, etc. 



The power to produce an acid is well shown by adding to neu- 

 tral or alkaline culture media a solution of litmus. The change in 

 color due to the formation of an acid may be followed by the eye, 

 and comparative tests may be made to aid in the differentiation of 

 similar bacteria. 



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 : C 6 H 12 O 6 2(HC 3 H 5 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 previously 



