Laboratory Outline for General Bacteriology 



2. If milk is properly cooled with ice the numbers should 

 increase only very slowly. 50,000 is a high count for milk deliv- 

 ered the next day even in warm weather. 



3. For milk which must be a longer time in transportation 

 higher numbers may be expected. Milk with more than 1,000,000 

 bacteria has not been properly cared for. 



DIRECT MICROSCOPIC EXAMINATION OF MILK 



The advantages of this method are that it is very rapid, re- 

 sults being obtained within a few minutes and before the milk 

 from which the sample is taken is used. It is simple and requires 

 little apparatus. It also gives some idea as to the kinds of bac- 

 teria. It does not, however, distinguish between the living and 

 the dead bacteria. With milk of fairly good quality the numbers 

 cannot be easily determined as no bacteria can be found. Its 

 chief advantage is in the quick detection of milk of very poor 

 quality. 



The milk samples are taken and shaken as for plating. With 

 pipette calibrated to deliver 1-100 cc., transfer this amount of 

 milk to a glass slide. Spread over just 1 sq. cm. The slide may 

 have the square centimeter ruled on it or it may be placed over 

 glass or paper ruled in centimeter squares to serve as a guide. 

 Duplicate smears should be made on the same slide. Milk on the 

 exterior of the pipette should be wiped off before making the 

 smear. Pipettes do not need to be sterilized. They are rinsed 

 out with water between samples. The water remaining is rinsed 

 out in the next milk sample. Dry the smears without heat. Cover 

 smears with xylol for 1 minute. Drain and dry. Cover with 95% 

 alcohol for 1 minute or more to fix the film to the slide. Drain and 

 stain with a fresh aqueous solution of methylene blue for 10 

 minutes. Wash. If film is too dark decolorize slightly with alco- 

 hol. Slides may be examined at once or kept for later reference. 

 They are examined under the microscope using the oil immersion 

 objective. The bacteria in at least 30 fields in a square centimeter 

 should be counted. With the Leitz microscope there are about 

 4800 fields in a square centimeter ; with the Bausch & Lomb 5,400. 

 Since only 1-100 cc. of milk is used the number of bacteria is found 

 by multiplying the average number of bacteria per field by the 

 number of fields in 1 sq. cm. and this by 100. 



