56 EXPERIMENTAL DAIRY BACTERIOLOGY 



The plates should be examined daily and the final count 

 made after from five to six days' incubation. Often the pres- 

 ence of a large number of molds and liquefying bacteria neces- 

 sitates a much shorter period of incubation. The results may 

 be recorded in terms of the number of colonies per plate, but 

 for more graphic presentation should be converted into the 

 number of organisms that would fall into a 12-inch milk pail 

 during an average milking period (five minutes). 



Exercise. Each student will prepare ten pla'tes as described : (1) ex- 

 pose four of them in the stable at a time when dust is present in mini- 

 mum quantities ; (2) four when different barn operations producing 

 dust are in progress ; (3) the remaining plates are to be exposed in 

 the immediate neighborhood of an animal that is being brushed. 



Contamination from coat of animal. The udder and flanks 

 of animals, even under the best of stable conditions, are likely 

 to become soiled with manure, and unless elaborate precau- 

 tions are taken to remove the dirt, it is dertain to be an 

 important source of bacteria in milk, as during the milk- 

 ing process the movements of the animal and milker will 

 dislodge dust and accompanying bacteria not only from the 

 udder proper but from the flanks and sides of the cow. More 

 or less of this dirt and dust finds its way into the open 

 milk pail. 



The bacterial content of fresh manure is exceedingly high ; 

 hence very small quantities may increase the germ content 

 of the milk to a marked degree. 



The importance of these sources of contamination under 

 various conditions may be determined as follows : Plates are 

 prepared as in the previous exercise, and exposed under the 

 udder of a cow while this organ is being manipulated as 

 during the milking process. The culture plate should be ex- 

 posed by placing it in a milk pail which is held in the same 



