MILK 85 



marked for from eight to twelve hours ; at higher temperatures it 

 is scarcely perceptible. It is supposed by some authorities to 

 be an agglutinative rather than a germicidal property ; that is, the 

 bacteria may not actually be destroyed but gathered together 

 in clusters that will not readily separate. Thus a colony may result 

 from a group of bacteria instead of a single individual and a wrong 

 impression of decrease in number may be obtained. In any case 

 the action is comparatively feeble and is soon lost. 



Estimation of Bacterial Content. The statement made con- 

 cerning the bacteriological examination of soil and water is equally 

 applicable to the examination of milk ; by no known method can 

 the exact number present in a given sample be determined. Some 

 species grow slowly or not at all on culture media; some are 

 aerobes, others anaerobes ; some require body temperature, others 

 grow best at a lower temperature. Clusters of bacteria may re- 

 main attached even after vigorous shaking and develop as one 

 colony instead of several. Again, bacteria are not equally dis- 

 tributed in the milk and the sample may not be representative. 

 As the cream globules rise they carry along with them numbers 

 of the bacteria present, until finally four or five times as many 

 organisms may be found in the cream and upper layer as in the 

 lower portion. Unless the milk is thoroughly mixed before the 

 sample is taken this also may constitute a source of error. 



The method usually employed to estimate the number of bac- 

 teria present in a cubic centimeter of milk is the plating method 

 already described. The direct microscopic examination of a film 

 of milk has been advocated in order to eliminate the above sources 

 of error. A square centimeter is ruled on a glass slide and 0.01 

 c.c. of milk accurately measured and evenly spread over it. The 

 film is dried in the air, fixed with methyl alcohol, the fat dissolved 

 with xylol, and finally it is lightly stained with methylene blue. 

 The oil immersion lens is employed for the examination, and the 

 tube of the microscope is so arranged that the microscopic field 

 covers ^5- sq. mm. The average number of bacteria found in 

 each field is multiplied by 5000 to give the number of bacteria 

 contained in 0.01 c.c. of milk. When the results of the two methods 



