BACTERIA OF MILK 55 
ing at 15° C. (59° F.). Keeping milk at a tempera- 
ture of 15° C. (59° F.) or below, therefore, inhibits the 
least desirable type of lactose fermentation. The Bacil- 
lus bulgaricus requires a temperature of at least 25° C. 
(77° F.). 
The acid-forming bacteria are widely distributed, but 
according to Esten* the chief primary source of those of 
the Streptococcus lacticus type found in milk is the cow’s 
mouth. The organisms are present in the manger and on 
everything within reach of the cow’s mouth, also in the 
feces. The acid-formers of the Bacterium acidi lactici 
type are derived from sugar-containing grain and roots 
like corn, beets, and carrots, especially when they are cut 
into small pieces, packed and fermented (ensilage) ; they 
are also contained in the faces of cows (Weigmann). 
Milk vessels and utensils, and other things and places 
with which milk comes in contact become seeded with 
acid-forming bacteria when not properly cleaned and 
sterilized, and are usually the principal sources of con- 
tamination when these organisms are present in milk in 
excessive numbers. 
2. Gas-forming Bacteria.—Included in this group are 
the bacteria which ferment the lactose in milk and form 
gases in addition to acids. They also decompose the 
proteids to some extent, especially the casein (Fig. 7). 
Most of them belong to the large coli-aerogenes group 
of organisms. The milk is curdled in the form of a 
smooth, white, jelly-like curd, which is more or less per- 
meated with gas bubbles and is associated with some 
fluid. The aerogenes organisms form a greater quantity 
of acids and gases than the coli and they also form more 
1“ Bacterium Acidi Lactici and Its Sources,” Storr’s Agr. 
Expt. Sta. Bull. No. 59. 
