PART III 

 APPLIED MICROBIOLOGY , 



DIVISION I* 

 Microbiology of Air 



CHAPTER I 



THE MICROORGANISMS OF THE AIR AND THEIR DISTRI- 

 BUTION 



The atmosphere is not the normal habitat of bacteria, for growth and 

 multiplication cannot take place in it under ordinary conditions. The 

 phrase "microorganisms of the air" is therefore somewhat ambiguous. 

 The small size of microorganisms enables them to remain suspended for 

 considerable periods when physical forces have separated them from the 

 substrata on which they have developed. 



Microorganisms Present in the Air. — Molds, bacteria, and yeasts 

 are all found in the air under certain conditions. The first two are usu- 

 ally relatively abundant, the latter are less common. 



The common molds have adapted themselves for the most part to 

 wind distribution. They bear spores that are small in size and possess a 

 surface that is not readily moistened. These spores are resistant to 

 desiccation and light and remain viable for a considerable tine even 

 under unfavorable conditions. Furthermore, the fruiting ' bodies of 

 many, though not all molds, show a distinct negative hydrotropism, i.e., 

 the mycelium remains in contact with the moist substratum while the 

 threads which bear the spores rise at right angles to it. These latter are 

 so sensitive that they can detect slight differences in the moisture con- 

 tent of the air and grow in the direction which will bring the spores into 



T Prepared by R. E. Buchanan. 



247 



