142 Microbes and You 



milk, and this figure may even rise to 98% if spore-forming organ- 

 isms and heat-resistant species are present only in small numbers. 

 Total bacterial counts, as found by the Breed Smear to be dis- 

 cussed later, will reveal not only the total bacterial count, but the 

 smear will also point out whether leucocytes (pus cells) are 

 present— an indication that an infection exists in the animal from 

 which the milk was drawn. 



When preparing bacterial vaccines it is necessary to standardize 

 the number of organisms within prescribed limits. Since these 

 suspensions are to be employed as killed cells, a technic must be 

 available for determining the total bacterial count. 



BREED SMEARS 



The Breed Smear has its greatest application in the milk in- 

 dustrv. Bv spreading 0.01 ml. of milk over a 1 square centimeter 

 area on a clean slide, and staining the dried and fixed preparation 

 with an appropriate dye (usually methylene blue), it is possible 

 to count the number of cells present in the preparation and to con- 

 vert the information into terms of cells per milliliter of milk. 



Standardization of the microscope is necessary. This will de- 

 termine the area you are viewing when you look through the oil 

 immersion objective, which is the power of the microscope em- 

 ploved during such examinations. There are available slides upon 

 which are etched lines spaced 10 microns apart; such slides are 

 called STAGE micrometers. By focusing the oil immersion objec- 

 tive on these etched lines, the diameter of the microscopic field 

 can be determined. For example, if the field is found to be 16 

 stage micrometer spaces across, the diameter of the microscope 

 field is 160 microns ( 16 X 10 ) . You may recall that the area of a 

 circle is determined by the formula: Area = 7rr-, where pi is 3.1416 

 and the radius is one-half the diameter. 



To continue with the above example, the area of the microscope 

 field is 3.1416 X 6400 (the radius is 80 fx and it is squared by mul- 

 tiplying it by itself-80 X 80, or 6400). By multiplying these fig- 

 ures, the area of a single microscope field is 21,106 square microns. 

 Since the milk was spread over an area 1 centimeter square (100,- 



