365 
the amount of sugar decomposed and that the quantity of acid present 
; at any one time during the time of the fermentation is not strictly 
proportional to the amount of sugar decomposed, for the reason that 
a part of the lactic acid resulting from the decomposition of the 
If sugar is in all probability decomposed into other substances. Accord- 
i ing to this observer, 1,000 lactic bacilli decomposed in one hour an 
amount of sugar varying according to conditions from 0.00001 to 
0.008 milligram. 
The changes brought about in milk by micro-organisms are by no 
means confined to the production of lactic acid. In fact, as we have 
already seen, it is only by working with pure cultures of certain of 
the lactic acid organisms, such as the Streptococcus lacticus, etc., 
under proper conditions, that lactic acid alone is produced, and in 
the souring of milk, as this ordinarly takes place, a great many sub- 
stances besides lactic acid are produced in larger or smaller amounts. 
Among these may be mentioned acetic, butyric, and succinic acids, 
alcohol and gaseous substances, such as hydrogen and carbon dioxide. 
In addition to these substances may be mentioned the production 
of small amounts of substances having characteristic odors usually 
of a disagreeable character. 
It would seem from the recent work of Tissier and Gasching (20), 
carried on in Professor Metschnikoff's laboratory, that in the souring 
of milk, as this usually takes place, we have a more or less regular and 
definite sequence of changes, due to the growth and development in the 
milk of various species of micro-organisms which were always found 
by these observers to be present in the milk as it left the dairy. In 
the samples examined by them they found constantly bacteria and 
fungi. According to these authors the bacteria present in milk are 
divisible into two groups: 
First. Mixed ferments, including the proteolytic mixed, such as 
Staphylococcus, rather rare, and the peptolytic mixed, such as En- 
terococci, B. coli, B. acidi paralactici, and B. lactopropylbutyricus. 
Second. The simple ferments, including the simple proteolytic, such 
as Mesentericus, Subtilis, B. putrificus, and Proteus vulgaris; the 
simple peptolytic, such as Proteus Zukeri and B. foecalis alcaligenes. 
The fungi are oidium lactis, rhizopus nigricans, and in one case a 
lactose yeast. 
In sterilized milk these authors have found these organisms to pro- 
duce the following changes: 
The mixed ferments accomplish two principal fermentations in 
milk, the lactic and the butyric fermentations. The lactic fermenta- 
tion is brought about by enterococcus, less actively by B. coli and 
most actively and vigorously by B. acidi paralactici, which possesses 
a high order of resistance. It produces chiefly dextrolactic acid. 
The butyric fermentation is accomplished by only one species, viz, 
