180 MICROBIOLOGY 



(2) From external contamination. The more common of these 

 sources are the dirt that falls from the skin of the flank and 

 udder into the pail during the process of milking; the dirt 

 from the hands and clothing of the milker; improperly 

 cleansed milk utensils ; and the dust and dirt that fall into the 

 milking utensils before and after milking or carried by flies 

 and other insects. 



Kinds of bacteria in milk. The bacteria present in milk 

 are (1) Non-pathogenic bacteria. These are the usual organ- 

 isms that come from the udder and those that are brought in 

 with the dirt from without. They vary in genera and species 

 according to the conditions. 



(2) Pathogenic organisms. These are disease producing 

 microbes that get into the milk direct from the diseased udder, 

 such as the bacterium of tuberculosis, anthrax, and the virus 

 of foot and mouth disease, and secondly those that are intro- 

 duced from without such as the organisms of diphtheria and 

 typhoid fever. These latter gain access to the milk either from 

 the hands and clothing of those suffering from the disease or 

 having the care of the sick, or with infected water used in 

 washing the utensils or by means of flies. The attendants may 

 be "carriers" and consequently responsible for the infection 

 of the milk.* 



Number of bacteria in milk. The methods of determin- 

 ing the number of bacteria in milk are similar to those used in 

 other quantitative analyses. Ordinarily the milk is diluted 

 with sterile salt solution from 1 to 10 to 1 to 1000 or higher. 

 From these dilutions plate cultures are made with definite 

 quantities. The media commonly used for this work are 

 gelatin and agar. 



The number of bacteria in mixed milk varies from a few 

 hundred to many millions per cubic centimeter. The variation 



* The bacilli "carriers" or people who have recovered from the 

 acute attack of the disease but who still harbor the organisms pre- 

 sent a most difficult problem for the milk and dairy inspectors to 

 solve. It is believed by some sanitarians that to reduce the danger 

 of infection from this origin all market milk should be pasteurized. 



