When counting bacteria using the dilution method, why do we start with the most dilute sample, ending with the most concentrated?  this question was posed in the Enumeration lab for a microbiology.

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boblawrence | (Level 1) Associate Educator

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I assume you are referring to the dilution method for counting bacteria, and have reworded your question to more clearly reflect this.  I hope this is what you meant.

If one wants to count bacteria on a swabbed surface it is necessary to dilute the sample.  This is because most surfaces would contain so many bacteria that a swab smeared directly onto an agar plate, without dilution, would result in a heavy “lawn” of bacterial growth.

By using a diluted specimen, the bacterial growth will spread out, allowing you to count individual colonies.

The method involves taking a swab and inoculating it into a test tube of liquid growth support medium.

You then take a pipette and draw up a measured volume of the liquid specimen and add to it an equal volume of new medium, thus diluting the bacterial concentration to ½ the original.

This is repeated 3 to 5 times, forming concentrations of 1, 1:2, 1:4, 1:8, 1:16 and so forth.

You then dip a swab into each tube and smear on a separate plate for each.   You start with the most dilute specimen, and work toward the undiluted specimen.

Chose a plate that has a heavy but distinctive and countable distribution of colonies.  Count the bacterial colonies, and then multiply by the dilution factor for that specimen to come up with an estimation of the bacterial count for the surface area tested.

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