Plating and Counting 133 



Checks. With a sterile pipette, place 1 c. c. of 

 the dilution water in a sterile plate, and add the usual 

 amount of media. Also pour 10 c. c. of media 

 into a sterile, empty petri dish. If litmus is used, 

 make a third check with 2 c. c. of litmus and 10 c. c. 

 of the media. In this way the state of the material 

 may be determined. The following scheme from the 

 Iowa Bacteriological Laboratory Report will be of 

 value in estimating dilutions: 



After the diluted milk is in the petri dish, if the litmus is 

 wanted, add 2 c. c. with a sterile pipette. If not desired, this 

 may be omitted. Melt the tubes of agar or gelatin, cool and 

 maintain at a temperature of 40 C. Finally, add 10 c. c. of the 

 nutrient media, either agar or gelatin as desired, being careful to 

 pass the mouth of the tube through a flame before pouring it. 

 Give the dish a revolving motion, to mix the diluted milk and 

 media, and then allow it to harden. Agar should be incubated 

 at 37 C. for forty-eight hours, while gelatin needs a temperature 

 of 20 C. for five days. When ready to count, place the petri dish 

 over a counting-plate and, with a hand lens, count the number of 

 colonies. If a counting-plate is not available, lines may be made 

 across the bottom of the plate with a blue pencil, for marking 

 glass to aid in counting. Each colony represents an original 

 organism. Multiply the number of colonies by the dilution, and 

 the result is the approximate number of organisms in the sample. 

 Repeat this with six plates, and take the average of them as the 

 final count. 



GLASSWARE NECESSARY FOR PLATING ONE SAMPLE OF MILK 



9 petri dishes. 1 250 c. c. glass-stoppered 



3 1 c. c. (straight) pipettes. bottle. 



1 2 c. c. pipette. 1 graduated cylinder 100 c. c. 



1 liter flask (water). 5 feet glass tubing, width 10 m. 



1 250 c. c. flask (litmus). m. or metal box. 



2 150 c. c. glass-stoppered 9 test tubes (Board of Health) 



bottles. 1 counting plate. 



