286 GENERAL MICROBIOLOGY 



EXERCISE 8. TO DETERMINE THE NUMBER AND 

 TYPES OF MICROORGANISMS IN CHEESE 



Apparatus. Cheddar cheese; cheese trier (sterile) if 

 an uncut cheese is to be sampled; mortar, with pestle; 

 two knives, sterile; quartz sand; sterile filter papers about 

 6 and 8 cm. square; one dilution flask containing 95 c.c. 

 of 0.85% salt solution; dilution flasks, containing 90 and 

 99 c.c. sterile 0.85% salt solution; sterile 1 c.c. and 10 c.c. 

 pipettes; three tubes litmus lactose agar; three sterile 

 Petri dishes; sterile litmus milk tubes. 



Method. 1. Sterilize in the hot-air oven, 10 gms. of 

 sand in a mortar with a pestle. 



2. Using a red-hot knife blade, sear a portion of the sur- 

 face of the cheese. 



3. To weigh the cheese " aseptically," place the smaller 

 sterile filter paper upon the larger on the balance pan. 



h 4, With a sterile knife remove the inner portion of the 

 seared surface and obtain and weigh out 5 gms. of cheese, 

 using aseptic precautions. 



5. Transfer the cheese to the sterile mortar and grind 

 up well. 



8. Transfer the cheese and sand mixture with sterile 

 knife or spatula to the 95 c.c. dilution flask. Shake well to 

 free the sand from the cheese. 



Directions for making dilutions. In transferring with 

 a pipette a portion of the first suspension to other dilution 

 flasks the sample should be taken immediately after shaking 

 before the sand has settled. Settling may be avoided by 

 holding the pipette in a horizontal position until ready to 

 deliver the contents. The grinding material should be fine 

 enough to avoid clogging the pipette. 



7. Make and plate the following dilutions in litmus 

 lactose agar: 1 : 100,000; 1 : 10,000,000 and 1 : 1,000,000,- 

 000, if the cheese has been made recently. 



If the cheese is not perfectly fresh, use dilutions 1 : 100,- 



