What is a colony forming unit and how is it used to infer microbial numbers in a sample?

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A Colony Forming Unit (also known as CFU or cfu) is a measure commonly used in microbiology to determine the number of bacteria (or fungi) present in a sample that have the ability to multiply under controlled growth conditions.

If someone wishes to know the number of microorganisms present in a sample, one way would be to count all the microorganisms under a microscope. However, this would be time-consuming. An easier way is to dilute the sample and spread a small aliquot on a petri dish. Given suitable growth conditions, the microorganisms will grow into a number of small colonies. Assuming that each colony was formed from a single viable microorganism, we can estimate the total number of microorganisms present in a sample.

For example, 100 colonies are counted on a petri dish made by spreading 1 ml of a sample that was obtained after diluting the original sample in a 1:100 ratio. In such a case, the original sample contains 10,000 (= 100 x 100) microorganisms per mL or 10,000 CFUs per mL.

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