54 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
colonies. It ferments lactose more slowly and requires 
a higher temperature than the Streptococcus lacticus. 
Acetic acid is the principal product of the lactose fermen- 
tation, although lactic and succinic acids and gas are 
also formed. Gas bubbles are rather numerous in the 
curd and the fluid expressed from the latter is not always 
clear. The sour taste of the milk is frequently un- 
pleasant. The Bacterium acidilactici is regarded by some 
bacteriologists as a distinct species with strains showing 
differences resulting from environment and by others 
as the type of a number of species or varieties. The 
mastitis organisms, Bacillus Guillebeau (a and b), and 
some of the bacteria which produce slimy or viscid milk 
are closely related forms. 
Several varieties of long, thin, rod-shaped organisms, 
of which the Bacterium bulgaricus is a type, also form 
acid from lactose, but they operate so slowly at the usual 
temperatures that they are not a factor in the ordinary 
souring of milk. They are chiefly of interest because of 
their use in the preparation of the oriental milks (mazun, 
kefir, yoghurt). The organisms of the Bacterium bul- 
garicus group are usually present in ensilage and those 
found in milk are no doubt derived directly or indirectly 
from this source. 
The temperature at which the milk is kept has an 
important influence upon the character of the lactose 
fermentation. In milk kept at 15 to 20° C. (59 to 68° 
F.), the organisms of the Streptococcus lacticus type will 
grow much more rapidly than those of the Bacterium 
acidi lacticitype. The Streptococcus lacticus grows quite 
well at 15° C. (59° F.) and continues to grow at 10° 
C. (50° F.), while the Bacterium acidi lactici grows bet- 
ter at higher temperatures and practically stops grow- 
