PRODUCTS OF VITAL ACTIVITY. 137 
asmall quantity (one ése) of the microdrganism 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 4ceids.—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 aérogenes, and 
Bacillus coli communis. 
The formula showing the transformation of sugar into lactic 
acid is usually stated as follows : C,H,,O, = 2(HC,H,0O,). 
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 
