144 Microbes and You 



a time interval like seven seconds is another variable, even when 

 the individuals have a watch to look at. -The ideal situation would 

 be to have the shaking done with standardized machines, but that 

 is not possible for all laboratories. 



WRIGHT'S PROPORTIONAL COUNT TECHNIC 

 Wright in 1902 proposed a method of mixing a known volume 

 of bacterial suspension with a known volume of human blood, 

 spreading a loopful or two of the mixture on a slide, drying, stain- 

 ing, and finding the ratio of organisms to red cells in a given num- 

 ber of microscopic fields. Average healthy male blood contains 

 approximately five million red blood cells per cubic millimeter 

 (five billion per cubic centimeter, or milliliter), and female blood 

 averages about four and a half billion red cells per milliliter. For 

 more accurate determinations using Wright's technic, actual counts 

 should be run on the specific blood to be employed in the test. 



Since we know the number of blood cells per unit volume, the 

 ratio of bacteria to red cells allows a comparison to be made, and 

 the count of bacteria per ml. can be calculated. To give a simple 

 illustration, if the average blood cell count is found to be fifty per 

 microscopic field, and if in those same fields the average bacterial 

 count is found to be one hundred per field, there must be twice as 

 many bacteria as there are blood cells per unit volume. If we 

 know there are five billion blood cells per ml., there must be, in 

 this case, ten billion bacteria per ml. The principal error in 

 Wright's technic lies in the difficulty of securing a uniform distri- 

 bution of organisms and blood cells, but reasonably accurate re- 

 sults may be expected with this method for determining total bac- 

 terial counts in liquids. 



HEMOCYTOMETER METHOD 

 By using a special chamber, similar to the type employed by 

 clinical laboratories for determining blood cell counts, bacterial 

 suspensions can be examined and the number of organisms enu- 

 merated with an error estimated to be less than 10%. Some workers 

 claim that the error is less than 3%, and that this technic is ex- 



