THE BACTERIOLOGIC EXAMINATION OF MILK 413 



there is some polluting tributary which can be eliminated by 

 efficient inspection. If high numbers are the result of growth, 

 be this in utensils, or bottles, or during transportation, the types 

 of bacteria that are present are usually of harmless varieties. 

 On the other hand, the presence of a germ carrier may render a 

 milk of small bacterial content suspicious. ' 



Examination of milk for bacteria leads to the discovery that 

 one or more phases in the long journey of milk from producer to 

 consumer is not receiving proper care, but does not locate the 

 exact source of contamination and does not distinguish between 

 harmless and injurious organisms. Furthermore, the results ob- 

 tained by different workers and by different methods are far from 

 uniform. 



There are two methods of counting bacteria in milk, namely, 

 by the colony count and the count by direct microscopic examina- 

 tion. These methods have both advantages and shortcomings. 

 The enumeration of colonies is at present more commonly prac- 

 tised than the direct method of counting. Since it is important 

 for the control of milk-supplies to obtain results in as short a time 

 as possible, it is unfortunate that there is no possibility of obtain- 

 ing results by the usual plate method before a definite period of 

 incubation has elapsed. Consequently, the milk which is being 

 examined is consumed by the time results are available. It fol- 

 lows that enumeration of bacteria by the plate method cannot 

 serve the purpose of rating a single day's supply. The benefit 

 derived is the control of regular supplies. By making several 

 tests of a milk from a particular source the average quality and 

 the conditions at the dairy can be judged, but it is not sufficient 

 to make occasional tests. It happens not infrequently that a 

 milk suddenly contains more than the usual number of bacteria. 

 Exceptional conditions may be the cause, and it would not be 

 fair to judge the regular supply by these conditions. Tests should 

 be made at regular intervals and at all seasons. Bacterial ex- 

 amination is useful for detecting irregularities in the general sup- 

 ply, but its usefulness for the purpose of estimating the quality 

 of a single bottle or can of milk is limited. 



Laboratory methods of preparing plates for the bacterial enu- 

 meration in milk vary considerably, and it is largely due to this 

 fact that results are so widely divergent. This divergence is 

 sometimes so great that the same sample of milk would be classi- 

 fied under different grades. The shaking of the milk sample is 

 the first step that may lead to different results. If milk is shaken 

 violently the clumps of bacteria are broken up to a greater extent 

 than if milk is poured from one vessel to another. On the other 

 hand, violent shaking introduces the error of swelling the volume 



