BACTERIA IN MILK AND FOOD 249 



SOME BACTERIA FOUND IN MILK 



Fermentation of Milk. Lactic Acid Lactose. Fermenta- 

 tion of milk is due to the conversion of milk-sugar into lactic 

 acid. This can be accomplished by a number of different 

 bacteria, such as Bacillus coli, streptococci and staphylo- 

 cocci, which are apt to be present about the dairy. The 

 lactic-acid bacteria are commonly present in sour milk, and 

 are chiefly concerned with fermentation. There are several 

 varieties, but principally three groups. 



The first group, like the Streptococcus pyogenes, is called 

 the Bacterium lactis acidi group. Milk is curdled within 

 twenty-four hours without gas-formation. The milk has a 

 mild acid taste and agreeable odor. The curd is even, a 

 true lactic fermentation. 



The second group resembles the Bacillus coli Bacillus 

 lactis aerogenes. Indol and hydrogen sulphid often formed. 

 Milk curdles, but the curd shrinks. Not easily emulsified. 

 This fermentation undesirable. 



The third group, true lactic bacteria Bacterium bulgari- 

 cum; exclusively lactic acid; curd easily broken. 



Bacterium Acidi Lactici (Hiippe) . Belongs to the same 

 group as the Bacillus coli communis (see page 134). 



Synonyms. Bacillus acidi lactici; B. lactis aerogenes 

 (Escherich) . 



Origin. In sour milk. 



Form. Short thick rods, nearly as broad as they are long, 

 usually in pairs, resembling B. coli. 



Properties. Immotile. Does not liquefy gelatin. Breaks 

 up the sugar of milk into lactic acid and carbonic acid gas, 

 the casein being thereby precipitated. The fermentation of 

 milk produced by this group is offensive; taste undesirable. 

 Curd is firm. 



Stain. Does not take Gram. 



Growth. Rapid and abundant; is facultative anaerobic. 

 Grows at 10 C. Grows in all media and in absence of car- 

 bohydrates. 



Stab-culture. A thick dry crust with cracks in it forms on 

 the surface after a couple of weeks. 



