l6o LABORATORY GUIDE IN BACTERIOLOGY 



SECTION 9 



EXAMINATION OF MILK FOR MOLDS AND 

 YEASTS 



1. Prepare plates in beerwort agar from undiluted 

 milk and from dilutions of i : lo, and i : loo. 



2. Incubate these at 37° C. and examine for molds 

 and yeasts. 



SECTION 10 



EXAMINATION FOR LEUKOCYTES IN MILK 



Examination for leukocytes in milk is carried on in 

 many laboratories and the test was formerly considered 

 of much value. Recent work, however, has cast 

 doubt on its significance, since it has been found that 

 milk of unquestioned purity and soundness may contain 

 as high as 1,000,000 leukocytes per cubic centimeter, 

 and with modern methods it is not possible to distin- 

 guish between leukocytes and pus cells. Leukocytes 

 are always present in milk in variable numbers. Some 

 observers believe that the presence of leukocytes and 

 streptococci together in large numbers indicate pus 

 in the milk. The reliabiUty of this test, however, has 

 not been suflSciently demonstrated. 



EXERCISE I. stokes' METHOD 



1. CentrifugaUze 100 c.c. of milk. 



2. Smear the sediment on a cover glass. 



3. Examine under the oil-immersion lens and record 

 the number of leukocytes and streptococci. 



