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Procedure for Calculating CFU (Colony Forming Unit)


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#1 att

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Posted 10 March 2020 - 10:47 AM

Hi, I have counted bacterial colonies from three consecutive dilutions (10^-3, 10^-4 and 10^-5 each in three replicaitons) from food samples and found the following figures, 

10^-3: 205, 210,  250

10^4: 105, 110, 103

10^-5: 55, 40, 47

My question:

1. Which of the concentrations should I consider for cfu calculation (all the colonies are within the acceptable range - 30-300)?

2. Or should I take all the concentrations and in what way?

3. Please show me all the calculation procedure one by one,

 

Thank you. 

att. 



#2 SQFconsultant

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Posted 10 March 2020 - 01:06 PM

Good morning ATT - welcome!


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Glenn Oster
 
 
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#3 Charles.C

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Posted 10 March 2020 - 04:41 PM

Hi, I have counted bacterial colonies from three consecutive dilutions (10^-3, 10^-4 and 10^-5 each in three replicaitons) from food samples and found the following figures, 

10^-3: 205, 210,  250

10^4: 105, 110, 103

10^-5: 55, 40, 47

My question:

1. Which of the concentrations should I consider for cfu calculation (all the colonies are within the acceptable range - 30-300)?

2. Or should I take all the concentrations and in what way?

3. Please show me all the calculation procedure one by one,

 

Thank you. 

att. 

 

Hi att,

 

I assume all the data refers to the same original sample.

I assume the same volume of each dilution was pipetted onto petri plates.

I assume incubation was at approx 37degC for 2 days.

 

I regret that the data shows a serious inconsistency due to the fact that there should be an  approx. relationship in agreement with the relative dilutions..

So there is no way to tell which set of data (if any) is realistic.

 

It looks like an error has occurred in the methodology used.


Kind Regards,

 

Charles.C


#4 att

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Posted 11 March 2020 - 05:32 AM

HI, Yes the data are from the same sample. The sample was serially diluted and 0.1 ml of the three consecutive dilutions (10 to the power 3 up to 10 to the power 5) were plated in three replications and incubated at 37oC for 48 hrs. My question is, it might be simple to calculate if only one dilution was plated. But here the figures are from three dilutions which gave us counts that are in the acceptable range (30-300) and we need to calculate CFU/ml of sample from the data. How can we consider the three dilutions for the calculation?

Best Regards

att.



#5 Charles.C

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Posted 11 March 2020 - 08:19 AM

HI, Yes the data are from the same sample. The sample was serially diluted and 0.1 ml of the three consecutive dilutions (10 to the power 3 up to 10 to the power 5) were plated in three replications and incubated at 37oC for 48 hrs. My question is, it might be simple to calculate if only one dilution was plated. But here the figures are from three dilutions which gave us counts that are in the acceptable range (30-300) and we need to calculate CFU/ml of sample from the data. How can we consider the three dilutions for the calculation?

Best Regards

att.

 

Hi att,

 

I assume this is a pour plate procedure. If so, it is preferable to use 1ml on plate.

As per my post 3 the data is "suspect" and probably better to repeat using 1ml.

 

However if cannot repeat,  I suggest to apply this rule (Micro.examination of foods, da Silva, 2013) -

 

Rule 6 – Two consecutive dilutions with 25–250 colonies. Calculate the number of CFU of each dilution and compare the results.

6.a)   If one of the results is greater than the double of  the other, consider only the lower count

6.b)   If one of the results does not exceed the double of  the other, then both results must be considered, and  the  mean  value  should  be  presented  as  the final result

 

 

6a then gives a result of 2,200,000 CFU/gm  [ ((205+210+250)/3) x 10^(4) ]


Edited by Charles.C, 11 March 2020 - 02:57 PM.
emended

Kind Regards,

 

Charles.C


#6 Gway

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Posted 11 March 2020 - 04:19 PM

Where counts are all within the acceptable range use the result from the least dilute



#7 Charles.C

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Posted 11 March 2020 - 05:32 PM

Where counts are all within the acceptable range use the result from the least dilute

 

Hi Gway -

 

+ (sometimes) investigate why. :smile:


Kind Regards,

 

Charles.C





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