COUNTING COLONIES ON PLATES. 181 



Duplicate plates should always be made and the mean 

 of the number of colonies that develop upon them taken 

 as the basis from which to calculate the number of 

 organisms per volume in the original water. 



For example : From a sample of water, 0.25 c.c. is 

 added to a tube of liquefied gelatin, carefully mixed and 

 poured out as a plate. When development occurs, the 

 number of colonies are too numerous to be accurately 

 counted. 



One cubic centimetre of the original water is then to 

 have added to it, under precautious that prevent con- 

 tamination from without, 99 c.c. of sterilized distilled 

 water — that is, we have now a dilution of 1 : 100. 

 Again, 0.25 c.c. of this dilution is plated and we find 

 180 colonies on the plate. Assuming that each colony 

 develops from an individual bacterium, though this is per- 

 haps not strictly true, we had 180 organisms in 25 c. c. 

 of our 1 : 100 dilution, therefore in 0.25 c.c. of the 

 original water we had 180 X 100 = 18,000 bacteria, 

 which will be 72,000 bacteria per cubic centimetre (0.25 

 = 18,000, 1 c.c. = 18,000 X 4 = 72,000). The re- 

 sults are always to be expressed in terms of the number 

 of bacteria per cubic centimetre of the original water. 



Throughout this part of the work it is to be borne in 

 mind that when one refers to plates it is not to a set, as 

 in the isolation experiments, but to a single plate. 



Method of Counting the Colonies on the 

 Plates. — For convenience in counting colonies on 

 plates or in tubes, it is of advantage to divide the whole 

 area of the gelatin occupied by colonies iuto smaller areas 

 of exact size For this purpose several very convenient 

 devices exist. 



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