618 APPLICATION OF METHODS OF BACTERIOLOGY 



there developed on a plate or tube made with one drop too 

 many colonies to be easily counted, then the sample must 

 be diluted with one, ten, twenty-five, fifty, or one hundred 

 volumes, as the case may require, of sterihzed distilled water. 

 This dilution must be accurate, and its exact extent noted, 

 so that subsequently the number of organisms per volume 

 in the original water may be calculated. 



The use of a drop is not sufficiently accurate. The dilu- 

 tion should therefore always be to a degree that will admit 

 of the employment of a volume of water that may be exactly 

 measured, 0.25 and 0.5 c.c. being the amounts most con- 

 venient for use. 



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 organ- 

 isms 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 as 

 a plate. When development occurs the number of colonies 

 is too numerous to be accurately counted. One cubic cen- 

 timeter of the original water is then to have added to it, 

 under precautions that prevent contamination from with- 

 out, 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 perhaps not strictly true, we had 180 organ- 

 isms in 0.25 c.c. of our 1 : 100 dilution; therefore in 0.25 

 c.c. of the original water we had 180X100 = 18,000 bacteria, 

 which will be 72,000 bacteria per cubic centimeter (0.25 c.c. 

 = 18,000, 1 c.c. = 18,000X4 = 72,000). The results are 

 always to be expressed in terms of the number of bacteria 

 per cubic centimeter of the original water. 



