CHAPTER XII. 



THE BACTERIOLOGY OF THE AIR. 



MICRO-ORGANISMS are almost universally suspended in the 

 dust of the air, their presence being a constant source of con- 

 tamination in our bacteriologic researches and occasionally 

 a menace to our health. 



Such aerial organisms are neither ubiquitous nor equally 

 disseminated, but are much more numerous where the air is 

 polluted and dusty than where it is pure. The purity of the 

 atmosphere bears a distinct relation to the purity of the soil 

 over which its currents blow. 



The micro-organisms of the air are for the most part harm- 

 less saprophytes taken up and carried about by the wind. 

 They are almost always taken up from dry materials, ex- 

 periment having shown that they arise from the surfaces of 

 liquids with much difficulty. Not all the micro-organisms 

 of the air are bacteria, and a plate of sterile gelatin exposed 

 to the air for a brief time will generally grow molds and yeasts 

 as well. 



In some cases the bacteria are pathogenic, especially where 

 discharges from diseased animals have been allowed to collect 

 and dry. On this account the atmosphere of hospital wards 

 and of rooms in which infectious diseases are being treated is 

 more apt to contain them than the air of the street. How- 

 ever, because of the expectoration from cases of tuberculosis, 

 influenza, and pneumonia, which is often ejected upon the 

 sidewalks and floors of public places, the presence of occa- 

 sional pathogenic bacteria is far from uncommon in street- 

 dust. 



Giinther points out that the greater number of the bacteria 

 which occur in the air are cocci, sarcina being particularly 

 abundant. Most of them are chromogenic and do not liquefy 

 gelatin. It is unusual to find more than two or three varie- 

 ties of bacteria at a time. 



To determine whether bacteria are present in the air or 

 not, all that is necessary is to expose a film of sterile gelatin 



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