CHAPTEE XIV. 



METHODS OF EXAMINING AIR, SOIL, WATER AND OTHER 

 FLUIDS FOR BACTERIA. 



THE general principle underlying the examination of air, soil, 

 water, and other fluids is that of mixing a definite amount of these 

 substances with a suitable culture medium, pouring plates or Petri 

 dishes, and studying the developing colonies of bacteria, yeast cells, 

 moulds, etc., as to species and numbers of colonies developed. In 

 the case of substances like soil, water, milk, and other fluids a small 

 definite amount can be taken directly and mixed with the culture 

 medium. In the case of the air, a known volume must be aspirated 

 either into a culture medium or into a sterile bland substance which 

 is subsequently mixed with a culture medium. 



The methods of examination, however, vary a good deal according 

 to the particular object in view. If a soil is to be examined for the 

 presence of tetanus, anthrax or malignant oedema bacilli, some of 

 the material is inoculated directly into susceptible animals (mice, 

 guinea-pigs, etc.) without first resorting to cultural methods. Like- 

 wise, in the examination of air for tubercle bacilli (first extensively 

 carried on by Cornet) the dust which is aspirated with the air or 

 which has settled spontaneously from it must be secured and inocu- 

 lated directly into guinea-pigs. 



FIG. 104 



Wolffhtigel's apparatus for counting colonies. 



Counting the Colonies. In the examination of air, soil, water, milk, 

 etc., for microorganisms (bacteria, yeast cells, moulds, etc.) it is often 

 desirable to obtain as exact a numerical estimate as possible. This 

 is accomplished after proceeding as indicated above by counting the 

 number of colonies developed in the Petri dishes. 



Wolffhugel's Counting Apparatus. Colonies are generally counted 

 with the aid of a Wolffhugel counting apparatus, which consists of a 



