274 The Phagocytic Power of the Blood 



The Bacterial Suspension. This is prepared like the 

 similar suspensions used for determining agglutination, but 

 with greater care, since the bacteria taken up by the cor- 

 puscles are to be counted, and any variation in the number 

 of bacteria with which they come into contact may modify 

 the count. It is also necessary to avoid all clumps of bac- 

 teria for the same reason. 



The culture is best grown upon agar-agar for twelve to 

 twenty-four hours, the bacteria in young cultures being more 

 easy to separate than those in old cultures. Such a culture 

 may be taken up in a platinum loop, transferred to a test- 

 tube containing some 0.85 per cent, sodium chloride solution, 



Fig. 77. Grinding bacteria (Miller). 



and gently rubbed upon the glass just above the fluid, 

 allowing the moistened and mixed bacterial mass to enter 

 the fluid little by little. 



If the culture be older or of a nature that will not separate 

 in this manner (tubercle bacillus), it may be necessary to 

 rub it between two glass plates (Fig. 77), or in a small agate 

 mortar with a drop or two of salt solution, other drops being 

 added one at a time, until a homogeneous suspension is 

 secured. Such clumps of bacteria as may remain in the 

 suspension are easily removed by whirling for a few seconds 

 in a centrifuge. 



The next step is the standardization of the suspension. 

 Wright recommends for this purpose and for the standard- 

 ization of the bacterio-vaccines that the number of bacteria 

 shall actually be counted. This he does by mixing one part 

 of the bacterial suspension with an equal volume of normal 



