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# calculations of cfu/ml

calculation of total cfu

5 replies to this topic

### #1 Dima

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Posted 27 November 2013 - 11:13 AM

Hello all

I have already finished analysing all my samples for different types of bacteria.

So now i have calculated the cfu/ml for each dilution i.e. 1/10, 1/1000 and 1/100000  for bacteria in each sample, I have the cfu/ml of a selected bacteria for each of the dilution stated earlier. what is the next step? how can I interpret results when i have 3 values? do i take the average of  the 3 dilutions cfu values?? and present it or there is a way to do it and am not aware of?

I appreciate a lot your help and advice,

D.

### #2 bob1

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Posted 27 November 2013 - 12:20 PM

So how different are the values?  If you have done the experiment correctly, the values for each cfu calculation should be pretty similar, though counts from plates where the numbers of colonies are low (about 30 or fewer) get more and more inaccurate.  Ideally what you would do is choose a dilution and do that in triplicate, then average.

### #3 Dima

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Posted 27 November 2013 - 12:42 PM

Many thanks for the reply!!

for example I had averaged the 1/1000 and 1/100000 counts since i had duplicates of each and I got 220 cell counts (1/1000) and 28 (1/100000)

220 28

then i got the cfu/ml for each . so i choose from one of the dilutions if i understand you well??

### #4 bob1

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Posted 27 November 2013 - 03:52 PM

Yes, I would go with the plate with a higher number of colonies, as this is more statistically accurate .

### #5 Phil Geis

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Posted 28 November 2013 - 12:43 PM

Folks typically take counts from plates of counts between ~25 and 250 (e.g. see https://mc.usp.org/s...61MicroEnum.pdf )

### #6 Dima

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Posted 28 November 2013 - 02:51 PM

thankssssss you are always a great help in this forum!!