New York Agricultural Experiment Station. 897 



in relation to health can be identified at once. The method also 

 shows the number and kind of cells in the milk, some of which may 

 indicate the sanitary quality of the milk. On the other hand, the 

 samples used for direct microscopic work are very small, are some- 

 what difficult to measure accurately, and may not represent the milk 

 quite so well as larger samples would do. It is believed, however, 

 that the rapidity of the method makes it possible to duplicate 

 samples so extensively that the small size need not interfere with 

 the accuracy of the work. An important objection is, however, 

 that all bacteria are counted, whether living and active at time 

 of sampling, or already dead and harmless. 



By the plate method no dead organisms are counted, since living 

 germs alone can grow to visible colonies. The colonies on the plates 

 present differences in habit of growth that are quite characteristic 

 for certain types of bacteria and this makes identification of some of 

 them possible. By using different culture media, also, the plate 

 method may be used to prove the presence or absence of liquefying 

 bacteria, which cause some undesirable forms of milk decomposition, 

 or of acid-formers that sour the milk. If particular types or species 

 of bacteria are desired for special study, the plate method must be 

 used to allow selection, isolation and the making of " pure cultures." 

 Against the plate method we must place the length of time required 

 before the count can be made — two, five, or even seven days in some 

 cases before we can be sure that colonies have developed about all 

 possible centers. The expense for apparatus — sterilizers, incu- 

 bators, etc. — and the outlay involved in securing proper conditions 

 to prevent outside contamination is large with the plate method; 

 and the manipulations are so delicate that only trained bacteriolo- 

 gists can do the work successfully. The hand lens used is not 

 powerful enough to show the individual bacteria; and these can not 

 be readily studied on petri plates under the compound microscope; 

 hence the information secured from colony-growth is all that is 

 immediately available. 



The numbers of organisms present, without 



Comparing special regard to the kinds, is the information usually 

 the methods, sought in bacterial examination of market milk; 

 and if the new method fails to give this informa- 

 tion accurately, its other advantages must count for little. 



To determine the fundamental reliability of the microscopic 

 method as compared with the now standard plate method, counts 

 were made by Mr. Brew, by both methods, in 450 samples of milk 

 from several different sources. In the main these samples were 

 from milk from four farms contributing to the supply of the city 

 of Geneva. 



The studies on the milk of these farms began with the taking of 

 samples of the milk each morning for ten weeks, as it was brought 

 from the farms to a central delivery station in the city. Samples 



57 



