Serial Dilutions of the Sample

Serial Dilutions of the Sample

	Serial Dilutions of the Sample If we take one ml of the original sample (say a known weight of soil mixed in known volume of sterile water to have a soil suspension which contains a microbial mixture) and add it to 9 ml of sterile water, it will give I: 10 or 10-1 dilution of original sample, i.e. the original sample has been diluted to 1/10th. similarly we may prepare 1: 100 (10'2), 1: 1,000 (10"3), 1: 10,000 (10'4) and so on dilutions of the original sample . Finally one ml aliquot of any dilution is added to a sterile Petri dish to which are added 9 ml of sterile, cool, molten agar medium. The dishes are incubated at suitable temperature. Within few days colonies of each kind of microbes grow in the dish. The number of colonies of each kind is counted. This number is then multiplied by the dilution factor to find the total number of cells per ml of the original sample.
This method is, therefore, also used for quantitative estimation of microbial cells in a known volume of original sample. Suppose, we use one ml of 10-3 dilution and 15 is the total number of cells per ml of this dilution. Then total number of cells per ml of the original sample will be 15 x 1,000 (dilution factor) Le. 15,000 cells. Serial dilution can also be used for a pure culture and is thus called serial dilution method. A pure culture may be obtained by serially diluting the sample with sterile water to the point of extinction in numbers of cells. This method is used only if the organism to be isolated is present in large numbers in the mixture. Suppose we have a bottle of any sample containing 10 ml of the soil sample in sterile water, and it contains 1000 microorganisms, which means 100 microbes per ml. If we take out one ml of it and mix it with 9 ml of sterile water, we then have 100 microbial cells in 10 ml, or 10 microbes per ml (Le. 1:10 or 10-1 of original sample).
If we add one ml of this to another 9 ml of sterile water, then each ml would contain a single microorganism. This is just what we want. If we use one ml of this final dilution to inoculate some sterile medium, and we obtain growth, this presumably came from one cell and is pure.

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