56 BACTERIOLOGY FOR NURSES 



As sixty-three squares would cover the standard Petri dish the 

 number of colonies in one square multiplied by sixty-three would 

 give the total number present if they were evenly distributed 

 throughout the medium. Since this rarely occurs it is best to 

 count ten representative squares and multiply the resulting num- 



FIG. 19. Wolffhiigel Counting Plate. 



ber by 6.3. This is in turn multiplied by the dilution used to give 

 the number of bacteria present in the material examined. Briefly 

 it may be expressed thus : 



Number of colonies in 10 squares X 6.3 X dilution 



= Colonies developed from 1 c.c. of material plated. 



The above method is very satisfactory when only one species of 

 bacteria is present and the medium and surrounding conditions 

 are known to be suitable to their growth. If on the other hand 

 the material contains a number of different species, it is not likely 

 that the optimum conditions for one will be the optimum condi- 

 tions for all, and many organisms may not develop into colonies 



