How to calculate CFU (colony forming unit)

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How to calculate CFU (colony forming unit)

Colony forming unit is a measure or count of viable cells unlike direct microscopic methods for example hemocytometer where all cells are counted either viable or non-viable. The results are read as CFU/mL (Colony Forming Unit per millilitre) for the liquid cultures, whereas CFU/g (Colony Forming Unit per grams) for the solid cell cultures. Colony Forming Units can be calculated by using method available named miles and misra.

The method:
First of all if the culture is broth, dilutions are made and suitable dilution is selected for spreading and colonies are counted on the plate that has been spread with the selected dilution and then using this formula the colony forming units are counted:
Bacteria per mL= (number of colonies present on the plate) x (dilution factor)/volume of culture plate

EXAMPLE PROBLEM:
5ml of Bacterial Culture is added to 45ml of sterile diluent. From this suspension, two serial, 1/100 (10-2) dilutions are made, and 100µL is plated onto agar plate. After overnight incubation, 150 colonies were counted on the plate. Calculate CFU/mL of the original Sample?

SOLUTION:
First thing that we need to find out is the dilution of the original sample or how much is the sample is diluted.
Here in this problem the original sample or the bacteria culture was first mixed in a diluent, hence we need to find out this dilution factor first
That is: Final Volume/Sample volume
Final volume = Volume of bacterial culture added to diluent + Volume of diluent
= (5 + 45) mL
= 50mL
Sample Volume = 5mL
Dilution factor = 50/5
=10/1
= 101

Now, the two serial dilutions of 1/100 i.e. 10-2 were made
Hence,
Total Dilution Factor= 101 x 102 x 102
= 105
Now,
CFU/mL= (Number of bacterial colonies counted on plate x Dilution Factor) / Volume of culture plate
CFU/mL= (150 x 105) / 0.1
= 1.50 x 108

So, total colony forming unit = 1.5 x 108 per mL
Now we may want to convert the values to Log value:
For the we need to take log of the values obtained i.e. Log(1.5 x 108)
The value of base 10 Logarithm of 1.5 x 108 concentration comes out to be 8.1760912591

Significance of Colony Forming Unit calculations:
Colony Forming Unit is a unit used in microbial counting to estimate the number of viable microbes in a sample. Viable simply means the microbes that are able to divide and are alive.
This method lets us count only the live cells unlike other methods that counts number of cells irrespective of the viability of the cells.

By |2018-02-27T06:31:45+00:00February 21st, 2018|Fermentation, Food Microbiology, Molecular Biology, Plant Tissue Culture|Comments Off on How to calculate CFU (colony forming unit)

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