The Air We Breathe 255 



petri dishes to the atmosphere for varying periods of time. While 

 this is not a quantitative method, the flora can be evaluated quali- 

 tativ^ely by employing media designed to encourage growth of 

 specific organisms. A blood agar medium, for example, will favor 

 the growth of fastidious organisms, including certain streptococci. 

 Zones of clearing adjacent to colonies growing on a blood agar 

 medium represent a reaction known as hemolysis (destruction of 

 blood cells). The type and the degree of blood change has some 

 diagnostic significance to a trained observer. If the zone appears 

 green, this reaction is called alpha hemolysis. So-called green 

 streps fall into this category. If a colony exhibits a clear zone 

 with a punched-out appearance, this is beta hemolysis, and if no 

 visible change in the blood can be seen around a colony growing 

 on a blood agar plate, the reaction is called gamma. 



Tomato agar will encourage the growth of yeasts and molds, 

 while such selective media as Endo agar and eosine methylene 

 blue agar can be employed to evaluate the gram negative flora of 

 the atmosphere. By exposing several different types of media to 

 the atmosphere, it is possible to accumulate considerable data 

 pertinent to aerobiology. 



QUANTITATIVE AIR ANALYSIS 



A complete study of the microbiology of the atmosphere should 

 include quantitative determinations, and since the pioneer work of 

 Pasteur a number of published reports have advocated the use of 

 simple and ingenious devices to accomplish these ends. It is 

 desirable for surgeons to perform their operations in an atmosphere 

 where bacterial numbers are reduced to a minimum. Ultra- 

 modern hospitals provide their operating rooms with washed or 

 filtered air pumped into the room through air-conditioning systems. 

 It is amazing, however, what fine surgery can be accomplished 

 with relatively little post-operative infection in military hospitals 

 set up in tents out in open fields. While it may be true that the 

 air in these installations is not as clean as one would expect to 

 find in permanent hospitals, the standard aseptic practices em- 

 ployed can do a great deal to minimize infections. 



