274 GENERAL MICROBIOLOGY 



sediment tester; cotton disks for sediment tester; clean 

 empty milk bottle; one pint bottled milk from each of 

 several miscellaneous sources. 



Note. The same sample of milk must be used for A, B and C. 

 Proceed with tests in the order given. 



A. DETERMINATION OF MICROBIAL CONTENT 

 OF MILK 



Method. 1. Shake the sample in the bottle vigorously. 



2. Plate the dilutions 1 : 100,1 : 10,000 and 1 : 1,000,000 

 in litmus lactose agar. 



3. Place the plates at 25 C., and proceed with the 

 microscopic sediment test. 



4. Count the plates at the end of five days and estimate 

 the number of bacteria per cubic centimeter and the relative 

 proportion of acid to other types of colonies. 



5. Determine the morphology of the organisms making 

 up the colonies of each type and compare with the findings 

 in the microscopical sediment test. 



6. Are all organisms present microscopically? Explain 

 your results and draw conclusions. 



B. MICROSCOPIC SEDIMENT TEST 



Method. 1. Mix the milk well and warm about 30 c.c. 

 to 60 to 70 C. 



2. Place 10 c.c. of this well-mixed, warmed milk into 

 each of two sedimentation tubes. 



3. Place one tube on each of the scale pans and balance 

 by adding more milk to the lighter tube. The tubes must 

 be equal in weight or they will throw the centrifuge " off 

 center." 



4. Centrifuge in a machine designed for this purpose for 

 five minutes, till a more or less considerable compact sedi- 

 ment separates out. 



6. Pour or pipette off the milk above the sediment. 



