METHODS USED IN CULTIVATION OF BACTERIA 



195 



If dishes of an unusual size are employed, the square area must 

 be ascertained by measuring the radius and multiplying its square 

 by TT (w X R 2 = area) (IT = 3.141592). 



The number of bacteria in a given volume of a suspension can 

 be estimated by a variety of methods without cultivation. The one 

 most commonly used is the method developed by Wright, which 

 consists in mixing a small measured amount of the bacterial suspen- 

 sion with an equal volume of a red blood cell suspension in which 



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FlG. 26. WOLFFHUGEL COUNTING PLATE. 



the number of erythrocytes per cubic millimeter are known. Smears 

 are made and the relative number of bacteria and red blood cells 

 per one or two hundred fields are counted. A simple calculation 

 can then be made. This method has been described in the section 

 dealing with opsonin technique. 



Another useful technique is the direct counting of dilutions of 

 the bacterial suspension, unstained or stained with methylene blue, 

 in a modification of the Thoma Zeiss counting chamber, known as 

 the Helber chamber. 



