372 MILK BACTERIOLOGY 



which is being placed on the market. This is inilk produced from 

 herds free from tuberculosis and which are housed and cared for 

 under good sanitary conditions. Nearly as great care is taken in 

 its production as in that of certified milk. Some milk so produced 

 compares favorably with certified milk. 



Common milk is all milk not classified under the preceding 

 heads and may vary in microbial content from a few thousand to 

 many millions. The number and kind vary with the different 

 dairies which produce the milk and often with the city or state in 

 which it is produced, depending upon the nature of the law and the 

 strictness with which it is enforced. 



The number of bacteria reported by Hill and Slack for Boston 

 milk is given below: 



Per cent. 



Below 100,000 bacteria per c.c 42.00 



Between 100,000 and 500,000 per c.c 29.75 



Between 500,000 and 1,000,000 per c.c 9.75 



Between 1,000,000 and 5,000,000 per c.c 12.75 



Above 5,000,000 per c.c 5.00 



Uncountable plates 0.75 



Bacteria in Milk. Milk is one of the best foods for man. It is also an 

 excellent food for bacteria, as is seen from the facts that millions are 

 often found in a few drops, and in many cases the bacteriologist finds 

 it one of the best mediums on which to grow his laboratory cultures. 

 Therefore, milk should be protected from substances which contain 

 bacteria, especially the disease-producing ones. It is the methods 

 by which they enter and the speed with which they multiply that 

 we want to consider. But it should be stated at the outset that large 

 numbers of bacteria in milk indicate dirt, lack of refrigeration, or 

 age. It may or may not contain the germs of disease, but there is the 

 possibility. So milk with a high bacterial content is not necessarily 

 harmful, but when used as a food particularly for children is a 

 hazard too great to be countenanced, or, as stated by Conn : " Good, 

 clean, fresh milk will have a low bacterial count, and a high bacterial 

 count means dirt, age, disease, or temperature. A high bacterial 

 count is, therefore, a danger signal and justifies the health officer in 

 putting a source with a persistently high bacterial count among the 

 class of unwholesome milk." 



The number of bacteria occurring in milk varies with age, initial 

 contamination, the care with which it is handled and kept, tempera- 

 ture, and age. Milk may contain only a few or millions in each drop, 

 or some market milks at times contain as many, but not as danger- 

 ous, organisms as sewage. 



Initial Contamination. The source of bacteria in milk are: (1) 

 Intramammary, (2) introduced during milking process, (3) from 

 milk utensils, (4) from the use of special milk apparatus, (5) con- 



