New York Agricultural Experiment Station. 895 



In using this method a small, measured drop (.01 cubic centi- 

 meter) is taken directly from a well mixed sample of milk, spread 

 over a definite area of a clean glass microscope slide, and dried by- 

 gentle heat. Duplicate " smears " are usually placed on each slide. 

 When dry, the slides are placed in xylol (a colorless, liquid chemical 

 derived from benzine), which dissolves the fat. They are next 

 immersed in alcohol to harden, or " fix," the dried milk to the slide, 

 then in methylene blue to stain the bacteria and cells. A final 

 immersion in alcohol reduces the blue color somewhat, and brings 

 the microscopic objects out distinctly on a light blue field. 



The ceils, cell fragments and bacteria may now be easily studied 

 and counted under the microscope, the " fields " appearing somewhat 

 as shown on the plates. No printed reproduction, however, can bring 

 out the stained objects as they are revealed in the light-suffused 

 smear on the glass slide. 



By adjusting the tube of the microscope to a definite length and 

 using the proper eye-piece, each field examined will have a definite 

 area and will represent a fixed fraction of the whole smear and, 

 therefore, of the sample and, finally, of the milk itself. Several 

 fields are examined on each smear, and on one or more duplicates, 

 and the average count of cells or bacteria is taken as representative 

 of the milk from which the sample was drawn. 



STUDY OF BACTEEIA IN MILK BY THE MICROSCOPICAL METHOD. 



It is believed by the investigators at this Station, 



How bacteria that, for many purposes, the use of the compound 



in milk microscope for counting bacteria in milk is a much 



have been better method than the one now in common use. 



studied. The present method of study is an indirect one, 



and depends on the fact that bacteria are living 



organisms. It counts them, not as they are in the milk, but only 



after they have so increased in numbers that around each one, or 



around each invisible cluster of them, a " colony " has developed 



large enough to be seen by using a hand lens or even the unaided eye. 



The " technique," or detailed scheme of operations, in this 



method requires the use of a sample of milk, drawn carefully so it 



shall fairly represent the larger quantity from which it comes, and 



dilution of this sample to separate the bacteria and make it possible 



for colonies to grow without overlapping and obscuring one another 



or being so numerous that it is difficult to count them. 



A definite portion of this diluted sample is then mixed with a 

 " nutrient medium " or food supply, which is a gelatinous, trans- 

 lucent, or almost transparent material, firm at ordinary temperatures 

 but fluid when warmed. This must be made sterile by heat so that 

 no bacteria or other living organisms shall be present except those 

 coming from the milk sample. By thoroughly and repeatedly 



