BACTEEIAL MILK ANALYSIS 245 



Shake the first dilution twenty-five times, then with 

 a fresh sterile pipette remove 1 c.c., put into the second 

 dilution water, rinsing the pipette to the mark as before ; 

 this gives a dilution of 1 to 10,000. Shake the second 

 dilution twenty-five times, then with a sterile pipette 

 remove 1 c.c., and put it into the petri dish, using care 

 to raise the cover only so far as necessary to insert the 

 end of the pipette. 



Taking a tube of agar from the water bath, wipe the 

 water from outside the tube with a piece of cloth, remove 

 the plug, pass the mouth of the tube through the flame, 

 and pour the agar into the plate, using the same care 

 as before to avoid exposure of the plate contents to the 

 air. 



Carefully and thoroughly mix the agar and diluted 

 milk in the petri dish by a rotary motion, avoiding the 

 formation of air bubbles or slopping the agar, and after 

 allowing the agar to harden for at least fifteen minutes 

 at room temperature place the dish bottom down in the 

 incubator. The practice of mixing the diluted milk 

 with the agar in the tube, leaving a certain portion of 

 the bacteria unplated, is -not recommended by the 

 Committee. 



Controls. Plating should always be checked by con- 

 trols. A blank plate should be made with each set of 

 milk plates for control of the water, petri dishes, 

 pipettes, etc. 



For control on technique of plating it is recom- 

 mended that for work on "market milk," duplicates be 

 made each day on several plates. 



" Certified milk" should always be plated in dupli- 

 cate, and where possible it is well to have one man's 

 work occasionally checked by another. 



Unless duplicate plates show as a rule approximately 

 the same count, the worker should see if there is error in 

 his technique. 



Backs are very useful for stacking the plates and to 

 prevent breakage. 



Plating should be done always in a place free from 

 dust or currents of air. 



In order that the colonies may have sufficient food 



