THE KINDS OF MICRO-ORGANISMS IN MILK 343 



To these groups soil bacteria might be added. That soil 

 bacteria may be present in milk and, in fact, probably are always 

 present, can hardly be questioned. They may be carried into the 

 stable with soil adhering to the cows after pasturing or wading 

 in water. However, they have so far received but little attention 

 at the hands of investigators, and probably are of small impor- 

 tance as inhabitants of milk. It should, furthermore, be remem- 

 bered that soil bacteria probably do not multiply in miUc, and 

 would be numerically negligible after the milk has been standing 

 for some time. 



Ayers and Johnson have grouped saprophytic bacteria in milk 

 as follows: 



1. Acid-coagulating — those bacteria that produce enough acid 

 to coagulate the casein promptly. 



2. Acid-non-coagulating — those that produce acid, but do not 

 coagulate the milk. 



3. Inert bacteria — those that produce no visible change in 

 litmus milk in two weeks. 



4. Alkali-forming bacteria — those that produce an alkaline re- 

 action in milk. 



5. Peptonizing bacteria — those that hquefy the proteins of 

 milk. 



This grouping was not designed to cover all types found, but 

 was the result of the authors' comparative study of the bacterial 

 flora in raw and pasteurized milk. Bacteria not forming colonies 

 on agar under aerobic conditions, molds, and yeasts were pur- 

 posely omitted from this classification. 



Lactic Acid Bacteria 



There are different conceptions of the meaning of the term 

 "lactic acid bacteria," inasmuch as some investigators include all 

 bacteria which are able to produce lactic acid from milk-sugar, 

 while others include only those which are responsible for the so- 

 called "normal" souring of milk, and some less common acid 

 fermentations. Still another group of investigators confine the 

 term "lactic acid bacteria" strictly to those kinds which are usu- 

 ally active in producing the "normal" souring of milk. 



There is really not much justification in grouping all bacteria 

 that produce lactic acid from milk-sugar as "lactic acid bac- 

 teria." This term has become so closely associated with the 

 souring of milk that we cannot logically include all organisms that 

 produce lactic acid from milk-sugar. Furthermore, the "normal" 

 souring of milk is not due to the formation of lactic acid alone. 

 Other acids are formed by certain bacteria that are almost invari- 

 ably associated with the "normal" souring of milk. 



