284 GENERAL MICROBIOLOGY 



REFERENCES 



MARSHALL: Microbiology (1911), pp. 319-321. 



ROSENAU: The Milk Question (1912), pp. 16, 37, 76, 105, 112, 120, 



128, 132, 138, 161, 185-230, 294. 

 WARD: Pure Milk and the Public Health (1909), pp. 71, 73, 74, 114- 



125, 



EXERCISE 7. DETERMINATION OF THE NUMBER AND 

 TYPES OF BACTERIA IN BUTTER 



Apparatus. Three tubes litmus lactose agar; litmus 

 milk tubes; fresh butter; sterile dilution flasks; three 

 sterile Petri dishes; sterile 1 c.c. volumetric (bulb) pipettes. 



Method. 1. Melt a small quantity of butter in a 

 test tube at the lowest possible temperature (not higher than 

 40 to 45 C.). Mix well. 



2. Using a warm pipette, transfer 1 c.c. of the well- 

 mixed melted butter to 99 c.c. of sterile (warm) salt solu- 

 tion. Free the pipette from fat by filling it with the dilu- 

 tion water several times. Use warm (50 C.) pipettes 

 and dilution flasks throughout so that the butter will not 

 stick to the pipettes and may be readily emulsified. 



3. Plate in litmus lactose agar, using dilutions 1 : 1,000, 

 1 : 100,000 and 1 : 1,000,000. 



Note. These dilutions may have to be changed. Look up the 

 average number of bacteria in the type of butter you are using and 

 make dilutions accordingly. 



4. Incubate the plates at 25 C. 



5. Weigh 1 c.c. of well-mixed melted butter and record 

 the weight in grams. 



6. Examine the plates after three to five days for acid 

 and other types of colonies. 



7. Count and record the number of bacteria per cubic 

 centimeter, also the types. Note the action of each type 

 on litmus milk. 



8. Estimate the number of bacteria per gram. 



