12. ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
EXERCISE 17 
PLATE CULTURES 
Melt four tubes of nutrient agar in boiling water. When thoroughly melted, 
cool to 42° to 45° C. While in a melted condition pour the agar into sterile 
Petri dishes. 
In pouring the agar from the tube into the Petri dish care should be taken to 
prevent contamination. When the melted agar is removed from the water bath 
the outside of the tube should be wiped with a cloth to remove the adhering water. 
When the cotton plug is removed to pour the agar, the mouth of the tube should be 
flamed to kill microorganisms.on the outside lip of the tube. In pouring the agar 
from the tube to the plate, the cover of the plate should be raised only on one side, 
and just sufficiently to admit easily the mouth of the tube. Care should be taken 
not to scrape the tube on the dish or its cover when pouring the agar. When the 
agar has been poured into the plate, lower the cover into place immediately, pick 
up the dish and gently tilt from side to side in order to make a uniform layer of 
agar over the entire bottom. Replace the plate on the table and allow the agar to 
harden. Care should be taken not to move the plate until the agar is thoroughly 
solidified and cold. 
Treat the agar plates prepared as follows: 
(1) Allow one to remain sterile as a control. 
(2) Remove the cover and allow one to remain exposed to the air of the 
laboratory for fifteen minutes before replacing the cover. 
(3) Remove the cover from one dish and touch the surface of the agar 
lightly with the tips of your fingers. 
(4) By means of your wire loop, streak some saliva on the surface of the 
agar of the remaining plate. Note carefully the technique of this operation as 
demonstrated. Especial care should be taken not to break the surface of the agar 
when streaking. 
After the plates are prepared and thoroughly hardened, they should be in- 
verted before incubation. 
Incubate at 37° C. After two days, make observations. 
