The Bacterial Suspension 



279 



rate 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, 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 hature that will not separate in 



Fig. 88.— :Grinding bacteria (Miller). 



this manner (tubercle bacillus), it m'ay be necessary to rub it between 

 two glass plates, or in a small agate mortar with a drop or two of 

 salt solutiouj other drops being added one at a time, until a homo- 

 geneous 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 standardization of the bacterio- 

 vaccmes that the number of bacteria 

 shall actually be counted. This he 

 does by mixing one part of the bac- 

 terial suspension with an equal volume 

 of normal Ijlood and three voluihes 

 of physiological salt solution. After 

 thorough mixing a smear is made upon 

 a slide, the smear stained, and the 

 number of bacteria and corpuscles in 

 successive fields of the microscope 

 counted untfl at least 200 red blood-corpuscles have been enu- 

 merated. As the number of red corpuscles per cubic millimeter 

 of blood is 5,000,000, the number of bacteria per cubic centimeter 

 can be determined from the results of the counting by a simple 

 arithmetical process. To facilitate the counting the eye-piece of 

 the microscope is prepared by the introduction of a diaphragm. 

 The prepared suspension must usually be greatly diluted before 

 using, but the reduction of bacteria is, of course, easily calculated. 



Fig. 89. — -Diaphragm 

 piece showing hairs in 

 (Miller). 



of eye- 

 position 



