48 A MANUAL OE BACTERIOLOGY 



SECTION X 



BACTERIA OF THE AIR 



In a strict sense there are perhaps no bacteria of the air, since 

 in that medium there is small chance for bacterial development, 

 yet there are species of bacteria commonly carried in air cur- 

 rents. These species are common on dead organic matter, and 

 are among those which cause contamination of culture media in 

 the laboratory. The greater part of the bacteria in the atmos- 

 phere probably are carried on particles of dust. Other things be- 

 ing equal, therefore, the more dust in the air, the more bacteria. 



Exercise 66. Relation of Bacteria to Atmospheric Dust 



Melt nine tubes of agar and pour each into a sterile Petri 

 dish. Replace the covers and allow the agar to harden. Label 

 one dish A and place it in the incubator for a control. Give the 

 other dishes consecutive labels and expose one in each of the 

 following named locations by removing the cover for exactly ten 

 minutes : , B, laboratory ; C, out of doors, at least 50 ft. from 

 any building ; D, basement ; U, room before being swept ; F, room 

 immediately after being swept with a dry broom ; G, room imme- 

 diately after being swept with a damp broom ; H, barn ; I, barn 

 after hay has been thrown down. Do not invert the covers while 

 making the exposure. Several students may cooperate in this 

 exercise. After the results are posted, each student will take 

 notes on the entire series. It will be noted that the results 

 obtained by this method are comparative, not absolute. 



Exercise 67. Determination of the Number of Bacteria in Air 



Fkankland, p. F. Phil. Trans. Roy. Soc. 178 B. : 113-152. 1887. 



Report of Committee on Standard Methods for the Examination of Air, Amer. 



Jour. Pub. Hyg. 20 : 346. 1910. 

 Rettgeb, L. F. Jour. Exp. Med. 22: 461. 1910. 



The following method, while not as exact as some, is simpler 

 and more likely to yield good results in the hands of students 

 beginning the subject. 



