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Average colony count

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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Average colony count

Postby SWEngineer » Thu Nov 24, 2011 12:58 pm

If I have an agar plate with some dilution, and have made two replicates of that plate. Saw that the number of colonies in replicate (1) was 31 and in replicate (2) 38. Now, in taking the average, we get 34.5

So, in this case, do I say that the average number of colonies is 35 colonies?

Thanks.
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Postby JackBean » Thu Nov 24, 2011 1:08 pm

First, it's colony forming units, AKA CFU. Second, you didn't dilute your sample at all? I wouldn't round the number.
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Postby JorgeLobo » Sat Nov 26, 2011 1:53 pm

Plese read the question. SWE reported that was countng colonies, not (yet) estimating the population in the undiluted sample, and asked re. what should be considered as the averge count of clonies - again not projected to the original sample.

USP projects statistical variation of about 0.5 log in plate counts. Do what you want here and be consistent. The difference between 34 and 35 is really of no significance.
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Postby michimichi » Wed Jan 25, 2012 4:20 am

get the average of the two and multiply it by the dilution u made= CFU/mL or g
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