CULTURAL TECHNIQUE 27 



Label six sterile culture dishes with the laboratory num- 

 ber of the sample of milk, date, and quantity of milk to be 

 placed iii each. Transfer to each of two Petri dishes 1 cc. of 

 the first dilution (1/100 cc. of the original milk), to two more 

 dishes 1 cc. of the second dilution (1/1000 cc.), and to the two 

 remaining dishes 1 cc. of the third dilution (1/10,000 cc.). ' 



In placing the diluted milk in the dish lift the cover at 

 one side, place the tip of the pipette on the bottom of the 

 dish and allow the desired quantity to run out. 



Do not remove the cover completely. 



Do not allow the pipette to touch the desk or any other 

 unsterile object. 



In a metal cup partly filled with water place seven tubes 

 of lactose agar. Heat over a Bunsen burner until the agar is 

 completely melted. 



Place a thermometer in the cup, cool to 45 C. by adding 

 cold water, and allow the tubes to stand five minutes at this 

 temperature. 



Remove a tube, wipe off the water, remove the cotton plug. 

 Flame the open end of the tube in a Bunsen flame. 



Lift one side of the cover of the culture dish, pour in the 

 culture medium, replace the cover, mix the medium with the 

 diluted milk by tipping the Petri dish from side to side, and 

 place the dish on a level surface. 



Incubate at 37 C. for forty-eight hours, s 



Count cultures and record results. 



Results obtained from duplicate plates should check within 

 15 per cent. 



Pour the seventh tube of melted agar into an empty, sterile 

 culture dish as a check. Incubate. If due care has been ex- 

 ercised and medium and glassware are sterile, no colonies 

 should develop in this culture. 



