89G Popular Editions of Station Bulletins of the 



shaking together the warmed liquid medium and the milk sample, 

 the bacteria are distributed somewhat evenly. The mixture is then 

 poured into a flat, circular, glass dish known as a " petri plate," 

 and spread evenly over the entire area. Usually one or more dupli- 

 cate plates are made from each sample, and the plates are placed 

 in an incubator to favor the growth of the colonies about each 

 bacterial center. The temperature of incubation must be quite 

 carefully regulated, for some bacteria are very sensitive and will 

 not grow unless all the conditions are right. After four or five 

 days, usually, colonies will have developed, presumably about each 

 germ or group of them, until they are large enough to be visible 

 under a hand lens of small magnifying power. Many of the colonies 

 can then be seen by the naked eye, but others will be of pin-point 

 size or smaller. All the colonies are counted on the whole plate or 

 a definite portion of it, and the number obtained multiplied by the 

 proper factor to account for the separation of the sample and amount 

 of dilution. The final figure is commonly spoken of as the number of 

 bacteria in the milk. This is never literally true, as some of the 

 colonies always develop, not from single bacteria, but from " clumps " 

 or unseparated collections of them. Moreover, a count made from 

 plates held at one temperature only does not show all the colonies 

 that might develop; for certain bacteria, like those accustomed to 

 life in the udder and the warmth of the animal body, will not grow 

 at low temperatures. By exposing the plates to such temperatures 

 for two days longer, additional colonies may be developed. The 

 opposite condition may also occur, and bacteria be present in the 

 milk that thrive only at temperatures lower than the one commonly 

 used for incubation. 



It will be seen from this condensed popular description, that the 

 " plate '' method of counting bacteria is complex and time-con- 

 suming; and it is dependable only in the hands of trained bacteriolo- 

 gists, equipped with elaborate and costly appliances. 



Do the two methods give equally reliable informa- 



Comparative tion regarding the number of organisms in milk? 



advantages This question will be discussed at some length 



of the two later, for the number of bacteria present is an 



methods. important index to the sanitary quality of market 



milk; but the two methods differ so materially on 



many other points that it is necessary to summarize briefly the 



advantages and disadvantages of each. 



The microscopic method is simple, comparatively inexpensive, 

 can be learned easily by any bright young man, and can be applied 

 successfully by men who are not necessarily trained bacteriologists; 

 it makes possible a report on the bacterial content of a sample of 

 milk within a very few minutes; and it shows not only the numbers 

 of bacteria, but also their forms. Through this feature of the 

 method, certain types of bacteria thought to be especially important 



