51 
3. After the plate has incubated in the desk from four to six days, examine 
it. Observe the kinds of bacteria and the relative number of each. Distin¬ 
guish the molds from the bacteria. Draw the hair with the bacteria and mold 
growths. 
4. Drop a cow hair into a tube of plain sterile milk. Leave the tube in the 
incubator from four to six days. Note the kind of fermentation that takes 
place. 
PART 3 
Contamination from Manure 
1. There is a suspension of one gram fresh cow manure in 999 cc. of sterile 
water on the desk. 
2. Place 1 cc. of the 1/000 manure suspension in a 99 cc. water blank and 
place 1 cc. of this dilution in each of the two Petri dishes, using a sterile 
pipette. With another sterile pipette place 1 cc. of the 1 to 100,000 dilution in 
a 9 cc. water blank and use 1 cc. of it for each of the two plates with a dilution 
of 1 to 1,000,000. 
3. When the agar is melted, cool it to 45° C. and pour the four plates. 
Place them in the incubator for 48 hours. 
4. Observe the different kinds of colonies. Notice the relative number of 
the different kinds. 
5. Count the colonies and estimate the number of organisms per gram of 
manure. Tabulate the results on the regular note paper as in Table VIII. 
PART 4. 
Contamination from Utensils 
Dilution 
Colonies 
Per plate 
Bacteria 
per gram 
Number and 
types of kinds 
1/100,000 gm. 
I 
I 
I 
II 
1/1,000,000 gm. 
I 
I 
II 
II 
1. Pour the milk out of the pail to be found on the desk and rinse the pail 
well with tap water. Drain as dry as possible. 
2. Pour in 100 cc. of sterile water, close and shake well. 
3. Place 1 cc. of this wash water in a sterile Petri dish. 
4. Rinse the can with a cupful of boiling hot water and drain dry. 
5. Pour in 100 cc. of sterile water, close and shake well. 
6. Place one cc. of this wash water in a sterile Petri dish. 
7. Boil 500 cc. of water in the can for 5 minutes. 
8. Place 1 cc. of this water in a Petri dish. 
9. Be sure that the Petri dishes are labeled with the number of the washing 
and desk number. Use lactose agar and pour each plate. 
10. Place the inverted plates in the incubator for 48 hours. 
11. Tabulate the results on the regular note paper, showing the relation of 
bacterial content of washings to treatment. See Table IX. 
