CHAPTER III 



BACTERIA IN THE AIE 



Methods of Examination of Air — Conditions of Bacterial Contamination of Air : 

 (1) Dust and Air Pollution ; (2) Moisture or Dampness of Surfaces : Bacteria 

 in Sewer Air ; (3) the Influence of Gravity ; (4) Air Currents. The Relation of 

 Bacteria to COj in the Atmosphere : in Workshops, in Bakehouses, in Rail- 

 way Tubes, in the House of Commons. 



The basis of the usual methods in practice for bacterially examiaiug 

 air is to pass the air over or through some nutrient medium. By 

 this means the contained organisms are waylaid, and finding them- 

 selves under favourable conditions of pabulum, temperature, and 

 moisture, commence active growth, and thus reveal themselves in 

 characteristic colonies. These are examined by the microscope and 

 sub-culture. Eeturns of the number of bacteria in the sample taken 

 may be made for the sake of information, but little or no conclusion 

 of value can be drawn from such data. The standard recognised in 

 Europe is the cubic metre or litre, and one may report, for example, 

 of the air of a room containing 500 or more germs per cubic metre. 



Methods of Examination of Air 



1. The Plate Method. — Koch adopted the simplest of all the culture methods, 

 viz. , exposing a plate of gelatine or agar for a longer or shorter time to the air of 

 which examination is desired. By gravity the suspended bacteria fall on the plate 

 and start growth. As a matter of quantitative exactitude, this method is not to be 

 recommended, but it frequently proves an excellent method for qualitative 

 estimation. It will be found in practice that nutrient agar is better for the purpose 

 than nutrient gelatine. Greater latitude is obtained both in point of temperature 

 and length of incubation, and the result is uncomplicated by the, at times, very 

 rapid liquefaction of the gelatine by liquefying organisms. Care should be taken 

 in preparing the plates to allow them to cool on a level surface, and at least 15 c.c. 



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