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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 they 
ordinarily can accomplish. 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 rap- 
idly, aided by the simultaneous action of the simple ferments. En- 
terococcus has been found to be the species predominantly producing 
inactive lactic acid, valerianic acid, and also always acetic acid. 
B. coli follows in its action, producing laevo lactic acid. Together 
these two organisms give an acidity to milk equivalent to 1.47 to 2 per 
cent of sulphuric acid. This degree of acidity arrests the action of 
the proteolytic ferments and brings on the coagulation or curdling 
of the milk. The B. acidi paralactici continues the destruction of 
the lactose, however, and gives rise to the true lactic acid fermentation, 
producing always dextrolactic acid. The medium having become 
favorable for its growth and development, the Bacillus lactopropyl- 
butyricus sets up its characteristic fermentation, producing always 
inactive lactic and also propionic and butyric acids until a total 
acidity of 4 to 6 per cent in terms of sulphuric acid is reached. This 
degree of acidity arrests all bacterial action. The fungi, oidium lactis, 
and rhizopus nigricans then intervene, however, and by oxidizing the 
organic acids and lactose and by effecting a further destruction of the 
casein again favor the growth and multiplication of those organisms 
whose development has been momentarily checked. It also appears 
from the work of Tissier and Gasching that the simple ferments alone 
can bring about the decomposition of the casein and its ultimate deriv- 
atives. They have further observed that the bacteria ordinarily con- 
cerned in the souring of milk are not in any way directly responsible 
for the digestive disturbances which occasionally result from the use 
of milk as a food. Under certain conditions, however, they may act 
as predisposing causes, but the accidents of botulism are due, accord- 
ing to these authors, to special species of organisms differing from 
those which are ordinarily concerned in the souring of milk. 
