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 contamination 

 in our bacteriologic researches and occasionally a menace to our 

 health. 



Such aerial organisms are neither ubiquitous nor uniformly 

 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 surfaces over which its currents 

 blow. 



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

 saprophytes taken up and carried about by the wind. They are 

 almost always taken up from dry materials, experiment having shown 

 that they arise from the surfaces of liquids with much diflSculty. 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 oidia as well. 



In some cases the bacteria are pathogenic, especially where dis- 

 charges 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. However, because of the expectora- 

 tion from cases of tuberculosis, influenza, and pneumonia, which is 

 often ejected upon the sidewalks and floors of public places, the pres- 

 ence of occasional pathogenic bacteria is far from uncommon in 

 street-dust. 



Gunther 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 varieties 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 on a plate or 

 Petri dish to the air for a while, cover, and observe whether or not 

 bacteria grow upon it. 



To make a quantitative estimation is, however, more difl&cult. 

 Several methods have been suggested, of which the most important 

 may be briefly mentioned: 



Hesse's method is simple and good. It consists in making a measured 

 quantity of the air to be examined pass through a horizontal sterile glass tube 

 about 70 cm. long and 3.5 cm. wide, the interior of which is coated with a film 



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