how do you do a plate count/serial dilution problem?

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Plate count helps us to calculate the degree of bacterial contamination of a sample. For plate counting the ist step is to dilute the sample as it contains billions of microorganisms and are not possible to count.

By dilutions the number of microorganisms decreases to a countable number. Then the diluted sample is placed on agar plate ( prepared by mixing growth medium with agar). Only 0.1 ml of the diluted sample is put on agar plate.

In the next step, the incubation of the plate is done in incubator where proper growth temperature is provided. Time of incubation depensd on the organism and the growth medium. After incubation period is complete, each viable cell grows into a visible colony. Then colonoco are counted to determine number of microorganisms present in original sample. Colony number should be low ranging from 30 to 300, so that counting becomes easier.

Finally to calculate the number of viable organisms that were present in the original sample, we have to take into account the amount of dilution and the amount of sample put on plate.

For example, if we had done two dilutions of the original sample i,e 1/100th of original sample and we got 30 colonies, then wenshowe multiply 30 by 100 and then by 0.1 ml ( as only 0.1ml was plated).

gladiator answered 1 year ago

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