148 



Microbes and You 



to insure that the number of colonies developing on the plates will 

 fall within the range of thirty and three hundred. Such a plate is 

 called countable. If more than three hundred colonies grow on 

 a plate, some bacteria are crowded out by the more aggressive or- 

 ganisms, and colony formation by the weaker bacteria may be 

 inhibited. 



1.0 ml. 



Bottle 

 No. 1 



1.0 ml. 



-- 1:100 



Dilution 

 (99 ML. Hp) 



Bottle 

 No. 2 



- 1:10,000 - 



Dilution 

 (99 ML. HjO) 



Petri Dishes 



Fig. 27. Method of preparing dilutions. 



The method of preparing dilutions is best explained with the 

 aid of a diagram, and the above shows how a sample of raw 

 milk might be diluted for plating purposes. The technic is not 

 restricted to milk, however. Each petri dish should be fully and 

 accurately labeled before diluted material is introduced into it. 

 Aseptic precautions must be practiced throughout the plating 

 process to minimize the entrance of contaminating organisms. 



