304 PRACTICAL DAIRY BACTERIOLOGY 



*No. 56. Quantitative Analysis of Miscellaneous Milk Sam- 

 ples. Obtain milk samples from several milkmen, learning, so 

 far as possible, the age of each sample of milk. These should 

 be collected in sterile bottles and kept on ice until they can be 

 experimented with in the laboratory. Dilute each sample of 

 milk 1,000 times and make agar plates from the final dilution; 

 incubate at 98. After 24 hours count the number of bacteria 

 in each plate and compare the samples of milk. 



No. 57. Qualitative Analysis of Miscellaneous Milk Samples. 

 With the specimens above collected it will be useful to attempt 

 a qualitative analysis, though this will be more difficult. The 

 dilution of the milk must be varied according to its age and 

 temperature. If it is fairly fresh only a few hours old a dilu- 

 tion of a thousand times is satisfactory. If it is 12 hours old, 

 a dilution should be, at least, 10,000; and if 24 hours old, it 

 should be as high as 100,000, or higher. The amount of dilu- 

 tion necessary may be determined by a direct microscopic study. 

 Make and stain a slide from each sample of milk, as directed 

 in No. 17. Count the number of bacteria per field. If the 

 average number per field is less than 10, dilute the milk 100 

 times. If it is as high as 100, dilute 1,000 times; and if still 

 higher, dilute in the same proportion. 



After diluting the milk make six litmus gelatin plates from 

 each sample. It is best to have three of these plates diluted 

 twice as highly as the other three. After cooling, place at a 

 temperature of 70. After two days, examine and determine 

 whether there are any rapidly liquefying colonies that are 

 likely to destroy the plates ; if so, make a study of the plates at 

 once. If possible, however, keep the plates for four days before 

 studying them. 



Study of plates. After the plates have grown (2 to 4 days) 

 study as follows: 



a. Determine total number of bacteria per c.c. of milk. Com- 

 pare with the total number found on the agar plates. 



