COLLECTING BACTERIA FROM THE AIR 39 



about 25. Observe and make notes on the appearance 

 of these Petri dishes after 24 hours. By this time it will 

 be observed that a number of spots of different sizes, 

 shapes, colors, etc., have formed on the surface of the 

 medium. Each of these spots probably consists of a 

 large number of the same species of organism, and is 

 called a colony. Besides bacteria, yeasts and mold spores 

 are constantly present in the atmosphere and drop on the 

 plate. These organisms, being of higher specific gravity 

 than the air, are constantly falling, or are carried hither 

 and thither by currents in the atmosphere. Small 

 particles of dust also may alight on the medium, and then 

 form a dark spot near the center of the colony. Dust 

 particles are usually covered with bacteria, possibly of 

 different species, and in this case the resulting colony may 

 not be composed of one and the same species. These 

 are not "pure cultures." 



Inoculate two or three colonies on agar-slants. 



NOTE. Agar-slants are prepared by allowing liquid agar to 

 solidify in a slanting position. 



Method oj inoculation 



1. Singe the cotton plug of a tube containing the 

 medium. It is imperative always to singe the cotton 

 plugs of tubes before opening them, whether these tubes 

 contain a culture or a sterile medium. The organisms 

 from the air are constantly falling on the cotton, and 

 bacteria may also be deposited on the cotton by handling 

 it with the fingers. If these organisms were not killed by 

 the process of singeing, they would drop on the medium 

 after removal of the stopper, and thus ruin a pure culture. 



2. Hold the tube (or, if a transfer is made, both tubes 

 side by side) between the thumb and the forefinger, so 



