359 
It would seem from the recent work of Tissier and Gasching (20), 
carried on in Professor MetschnikofPs laboratory, that in the sour- 
ing of milk as this usually takes place, we have a more or less regular 
and definite sequence of changes, due to the growth and develop- 
ment 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 
Enterococci, B. coli, B. acidi paralactici, and B. lactopropvlbuty- 
ricus. 
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 alcali- 
genes. 
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 fermen- 
tation 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, 
B. lactopropylbutyricus, which in order of sequence follows in the 
wake of the lactic fermentation. It is only dependent on it indi- 
rectly, however, since for the growth and development of this organ- 
ism in milk neither lactic acid nor lactates are required, but a hexose 
which is formed from lactose by the bacteria immediately preceding 
the growth of the butyric ferment. Thus the butyric ferment 
depends only indirectly for its action on the lactic acid fermentation. 
The simple ferments of milk have been found to peptonize and 
destroy the casein, but in symbiosis with the mixed ferments they 
are rapidly arrested in their action by the acid reaction of the medium 
and become powerless to effect those changes in milk which ordi- 
narily they can bring about. For the completion of these changes, 
therefore, the intervention of higher organisms is necessary. These 
are accomplished by the milk fungi, oidium lactis, and rhizopus 
nigricans. 
The progress of the souring of milk has been found by these observers 
to be always the same. The mixed ferments develop rapidly, aided 
