306 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



THE MILKER. No food material requires greater care and cleanliness 

 on the part of those handling it than does milk. All persons having to do 

 with the handling of this delicate food product should constantly keep 

 in mind that clean hands and clothing and extreme cleanliness in every 

 operation is very necessary if milk of good quality is to be obtained. 



GROUPS OR TYPES OF MICROORGANISMS FOUND IN MILK 

 AND THEIR SOURCES. 



In studying the types of bacteria which are found in milk, it is con- 

 venient to arrange them in groups based upon their action on the milk and 

 their effect upon persons consuming it. There are certain types of organ- 

 isms which are very troublesome to the milk dealer but which are not 

 injurious to the consumer. Other species which may be of little or no 

 significance from their action upon the milk are of greatest significance 

 from the standpoint of the consumer since most of the disease organisms 

 which may be carried by milk have no appreciable action upon it. Still 

 other forms are of but little importance to either the dealer or the con- 

 sumer and others are troublesome to both. 



GENERAL SIGNIFICANCE OF ACID-FORMING BACTERIA. Of all the 

 bacteria that find their way into milk, those that are able to ferment the 

 milk sugar, producing from it different kinds and amounts of acids, find 

 more favorable conditions for growth at ordinary temperatures, 15 to 45, 

 than do those belonging to other groups. Because of their greater rapid- 

 ity of growth and because of the inhibiting effect of their by-products upon 

 the other groups of bacteria, the acid types tend to predominate in milk 

 and the specific change which they produce, the souring, is of such 

 common occurrence that it is often looked upon as something inherent 

 in milk. 



GROUPS OF ACID-FORMING BACTERIA.* The acid-forming bacteria 

 that are constantly present in milk represent many kinds which differ in 

 morphology, in cultural characteristics, and in their products of fermen- 

 tation. They may be divided into four groups that vary greatly as far as 

 their importance in the handling of milk is concerned. If milk is pro- 

 duced under clean conditions and is kept at temperatures ranging from 

 15 to 35, the acid fermentation will be almost wholly due to a group of 

 bacteria closely allied to one of the pathogenic forms, Strept. pyogenes 

 (Rosenbach) . To representatives of this group, which is of the greatest 



* Prepared by E. G. Hastings. 



