MICROORGANISMS OF THE AIR. 



189 



SPECIES OF ORGANISMS IN THE AIR. Penicillium is the most common 

 mold isolated from the air. Next in importance are Mucor, Rhizopus, 

 and Aspergillus in the order given. In addition to these a considerable 

 number of species of hyphomycetous molds are occasionally found. 

 Torulce, but not true yeasts, are usually common. Bacteria are 

 either spore-bearing soil bacilli or cocci. Of the former, B. subtilis, B. 

 mycoides, and related forms are ubiquitous. Sarcina lutea and Sarcina 

 aurantiaca and certain other chromogenic cocci are to be found in almost 

 every plate exposed. Since the air does not have a true flora, the species 

 as well as the number of bacteria present must depend entirely upon the 

 character of the environment. 



