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 1 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 

 fasces. 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 fasces 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. 



