Procedure for Calculating CFU (Colony Forming Unit)
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.
Good morning ATT - welcome!
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.
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, 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) ]
Where counts are all within the acceptable range use the result from the least dilute
Where counts are all within the acceptable range use the result from the least dilute
Hi Gway -
+ (sometimes) investigate why. :smile: