18 
Exercise 8. The Distribution of Bacteria 
MATERIALS: 
A case of sterile pipettes 
One sterile, empty test tube 
Five sterile Petri dishes 
Five tubes of sterile, meat extract-peptone agar 
Soil suspension 
1. Label the five dishes near the edge of the covers. Be very careful not 
to handle the dishes any more than is necessary and remove the covers only 
when necessary. 
2. Mark the labels as follows: 
(а) “Air” 
(б) “Finger tips” 
(c) “Sterile” 
(d) “Soil suspension, onb drop” 
(e) “Tap water, one cubic centimeter” 
3. With a sterile pipette place a drop of the soil suspension in dish (d). 
4. Collect some tap water in a sterile test tube. 
5. With another pipette place one cc. of tap water in dish (e). 
6. Melt five tubes of agar (bacterial food in a jelly) in the metal cup by 
boiling them in water. When the agar is melted, cool to 45°C. by mixing cold 
water carefully with the hot water in the cup. Great care must be exercised 
not to cool the water below 45°C. At this temperature bacteria can be mixed 
with culture media without injuring them. 
FIG. 12—THE CORRECT METHOD OF POURING LIQUEFIED CULTURE MEDIA INTO 
PETRI DISHES 
After wiping the water from the outside of a tube of agar, remove its plug, 
flame the mouth of the tube in the gas flame and pour the agar into dish (a), 
(See Fig. 12). Tip the dish carefully from side to side to spread the agar 
over the entire bottom of the dish. Place the dish on a level part of the 
desk until the agar becomes solidified. In the same way pour agar into the 
other dishes. 
7. Expose dish (a) with the cover off to the air of the laboratory for 
twenty or thirty minutes. Leave the cover right side up by the side of the dish 
during the exposure. 
8. Carefully touch the finger tips and nails of one hand to the surface of 
the agar in dish (b). Keep dish (c) closed. 
9. When the agar is solid and dish (a) is exposed, invert the plates in the 
28°C incubator for one week. 
10. Rinse out pipettes used and leave on the desk to be sterilized. Wash 
tubes that contained agar, and invert in the desk. 
