Counting Colonies to Estimate Microbe Numbers Flashcards

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In microbiology, what is a colony on an agar plate?

A colony is a visible group of many microbes that has grown from one original cell or a small clump of cells.

Each colony comes from one living cell or a small clump, so the number of colonies shows how many microbes were in the volume spread on the plate.

You must incubate it at a suitable temperature so the microbes can grow into visible colonies.

It is the number of living microbes in one cubic centimetre of a sample, found by counting the colonies they form on an agar plate.

We dilute the sample to spread the microbes out so that colonies are separate and easy to count.

It is a step-by-step method where you repeatedly dilute a sample by the same factor to make a range of weaker samples.

Plates with about 30 to 300 colonies are usually easiest to count and give more reliable results.

If there are too many colonies they can merge together, so you cannot tell them apart and the count will be wrong.

With very few colonies, a change of one or two colonies makes a big difference to the result, so the estimate is less accurate.

You need the number of colonies, the dilution factor of the sample, and the volume of the sample you spread on the plate.

There were 80 × 1000 = 80 000 CFU/cm³ in the original sample.