CULTURAL TECHNIQUE 25 



the colonies will be abundant on one side and sparse or 

 absent on the other, making it difficult to secure a correct 

 approximation. In case the count to be made is not the 

 final one, or if the culture is to be used for other purposes, 

 the cover of the dish must not be removed during the 

 counting process. 



A number of plate cultures must be made from each 

 sample of milk. Not all of these are to be counted, but 

 only those having the most desirable number of colonies. If 

 plates have been made with 1/100, 1/1000, and 1/10,000 cc. 

 of milk, the colonies on the first plate of the series may be 

 so numerous as to render counting difficult or impossible. 

 The next plate, containing only 1/10 as much, may have 

 several hundred, while the final one of the series, made with 

 1/10,000 cc., should have only a few score of colonies. To give 

 reliable and consistent results plate cultures should show be- 

 tween forty and two hundred colonies. When the number of 

 colonies is much in excess of two hundred, inhibition of one 

 form by another takes place, and increases with an increas- 

 ing number of colonies. The maximum number of colonies 

 that will develop on a plate ranges from ten to fifteen thou- 

 sand. If the material used in preparing the plate culture 

 contained a greater number of organisms, the results will not 

 be accurate. If the colonies are few in number on the plate, 

 it is difficult to obtain concordant results. For these reasons 

 plates which do not approximate the number of colonies 

 considered as desirable should be discarded. The average of 

 all plates counted should be taken and the results expressed 

 as the number of bacteria per cubic centimeter in the milk 

 examined. 



Expression of results. Since it is impossible to determine 

 with absolute accuracy the number of bacteria present in any 



