78 EXPERIMENT STATIOlSr RECORD. 



in tine dairy situation. Recommendations to manufacturer, jobber, retailer, and 

 consumer relative to ascertaining the correct weiylit of butter are given. 



A comparison of the microscopical method and the plate method of count- 

 ing- bacteria in milk, J. D. Brew (New York State Sta. Bui. 373 (lOlJf), pp. 

 3S, pis. 2, figs. 2). — This bulletin gives the results of comparative studies be- 

 tween the direct microscopic method and the plate method of counting bacteria 

 as applied to market milk. The milk from 37 dairy farms was examined, 

 although the conclusions were based largely on the results obtained from 

 4 of these in which cases more extensive and detailed examinations were made. 



" The microscopical method used in this work consisted in measuring out 

 0.01 cc. of milk taken directly from a well-shaken sample by means of a 

 specially graduated and accurately calibrated pipette. The drop of milk was 

 deposited on a clean glass slide and spread over an area of 1 sq. cm. with a 

 stiff, straight needle. Duplicate smears of each sample were made on the 

 same slide. The milk was then dried by gentle heat which was obtained by 

 means of a level wooden surface over a steam radiator. Care was exercised 

 not to allow the smears to become too hot as this made them check and thereby 

 made satisfactory staining impossible. As soon as dry, the slides were placed 

 for a short time in a Coplin staining jar containing xylol to remove the fat. 

 They were then taken out and the surplus xylol about the edges of the slide 

 wiped off with filter paper. The smears were dried and then fixed to the slide 

 by means of a 95 per cent solution of alcohol. Immediately thereafter they 

 were stained from 2 to 3 minutes in Loeffler's methylene blue, after which they 

 were decolorized to a light blue in a 95 per cent solution of alcohol. The 

 counting was done under a 1.9 mm. oil-immersion lens." 



The results of these studies indicate that there is a well-defined relationship 

 existing between the direct microscopic count and the plate count. This is 

 more apparent in long series of examinations than in short series which renders 

 it impossible in individual samples to establish a definite ratio 'whereby the 

 results obtained by the one method can be interpreted on the basis of the other 

 method. " The relative differences between the two counts are greater where 

 the bacteria are few in number. In samples of milk showing plate counts of 

 less than 10,000 per cubic centimeter, the coimt by the microscope shows ap- 

 proximately 44 times as many individual bacteria, or 17 times as many when 

 the clumps and isolated bacteria are counted as units, individual bacteria in 

 the clumps not being counted. In those samples which give a plate count of 

 about 1,000,000 per cubic centimeter, the count made with the microscope 

 shows approximately 5 times as many individual bacteria ; or when the isolated 

 bacteria and clumps of bacteria are counted as units the number of these units 

 is slightly less than the number of colonies given by the plate method. 



It is concluded that the bacterial count obtained in milk by the direct micro- 

 scopic method is equally as good if not a better criterion of its bacterial con- 

 tent than the count obtained by the plate method, and has these decided prac- 

 tical advantages : " The number of bacteria can be shown in a given sample of 

 milk within a very few minutes. The apparatus required is less expensive 

 than that required for the plate method and the examinations necessary for 

 commercial grading can be made by men who are not trained bacteriologists. 

 The morphology of the bacteria present may be determined as well as the ap- 

 proximate number present. The microscope frequently shows many bacteria 

 present in samples of milk while agar plate counts from the same samples 

 are low." 



It is hoped that this method can be made of practical use to the milk dealer, 

 butter maker, and cheese maker as a means of grading milk according to its 



