248 MICROBIOLOGY OF AIR 



the driest situations. A slight current of air inU. detach the spores from 

 these structures and carry them long distances. 



Bacteria and yeasts lack the specific adaptations for wind distribu- 

 tion found in molds. The material upon which they have been growing 

 must be dried and pulverized before they can be blown about. Many 

 species produce spores or other resistant cells, and physiologica,lly are as 

 well adapted for air distribution as are the molds. 



Occurrence in the Air. — -Microorganisms are found free in the 

 air, attached to particles of dust, or enclosed in minute drops of water. 

 Mold spores are commonly free or in unattached clusters. Bacteria and 

 yeasts are usually associated with dust particles, frequently the pulver- 

 ized substratum on which they have been growing. Not all dust par- 

 ticles have living organisms attached. It has been computed that in 

 the air of London during a fog there is only one living organism for over 

 thirty-eight millions of dust particles. Microorganisms are some- 

 times sprayed into the air with water. Droplets containing bacteria 

 are thrown off in the saliva in coughing or in speaking, and from the 

 surface of fermenting liquids on which bubbles are bursting. When 

 the drop is small enough, the air currents keep it in suspension and the 

 water soon evaporates and frees the organism. This brings about the 

 condition first discussed, free bacteria in the air. The decrease in 

 weight and size incident to this loss of water probably accounts for the 

 fact that the so-called "infectious droplets" are sometimes carried for 

 considerable distances. 



How Microorganisms Enter the Air. — In comparatively few in- 

 stances do microorganisms possess mechanical devices for projecting 

 the spores or other cells into the air for wind distribution. Usually the 

 organism is passive and is freed only by air currents or by mechanical 

 agitation. Some molds, as has been stated, release their spores even in 

 the presence of moisture, so that complete desiccation is unnecessary for 

 their dispersal. Bacteria and yeasts, on the other hand, are not usually 

 given ofE from moist surfaces. Only when dry and pulverized can the 

 bacterial medium be readily blown about. Hansen found that in the 

 immediate vicinity of a heap of decaying malt, the air was comparatively 

 free from bacteria. Winslow has shown that sewer air is frequently 

 practically free from bacteria although the surface with which it comes 

 in contact teems with bacterial life. Mechanical agitation often throws 

 large numbers of organisms into the air. Moving hay and straw 



