l86 MICROBIOLOGY OF AIR. 



situations. A slight current of air will detach the spores from these 

 structures and carry them long distances. 



Bacteria and yeasts lack the specific adaptations for wind distribution 

 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 physiologically, 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 pulverized sub- 

 stratum on which they have been growing. Not all dust particles 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 sometimes 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 or- 

 ganism 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 off from moist surfaces. Only when dry and pulverized can the 

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

 mediate 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, groom- 

 ing animals, sweeping a floor or carpet will multiply the dust and bacterial 



