ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 45 
SECTION 16: BACTERIOLOGY OF AIR 
EXERCISE 64 
THE RELATIVE NUMBERS OF BACTERIA IN AIR 
Pour four tubes of nutrient agar into sterile Petri dishes and allow to 
harden. 
Expose duplicate plates as follows: 
(1) To the air of the laboratory for 5 minutes. 
(2) To the air of the laboratory for 20 minutes. 
Invert the plates and incubate either at 37° C. for 2 days or at room tempera- 
ture for 4 to 6 days. Count the number of colonies which develop but do not 
open the plates. 4 
Observe the types of colonies which develop. Make Gram stains and study 
microscopically. 
EXERCISE 65 
THE QUANTITATIVE BACTERIAL EXAMINATION OF AIR 
Draw ten liters of the laboratory air through a tube containing 5 cc. of 
sterile water. 
Plate the sample from the laboratory air in duplicate, using 1 c.c. amounts. 
Incubate the plates at 37° C. for 2 days, or at room temperature for 4 to 6 days. 
Tabulate the data obtained, recording the results in terms of the number of 
bacteria per liter and per cu. ft. of air. 
