Colony Forming Unit (CFU)

·           Is a measure of viable cell ( bacteria or fungal).

·          Colony forming unit (CFU) is a unit, used for estimation of viable cells.

·          Colony means one cell or  a group cells.

·          Cell counting using ‘heamocytometer’ or ‘colony counter’.

·          Where all cells, dead or living are counted, but cfu measures only viable cells.
         This unit used to reports the bacterial density.
 
    Alternative parameters :
       Instead of CFU, the parameters MPN (Most probable number) and MFU (Modified Fishmon units)can be used, MPN correlates with CFU.
     MFU also takes in to account of bacteria in VNBC (viable but non-culturable) mode.

      CFU = observed cfu x Volume to be reported  /  volume to be analyzed

        

·         Example :

  CFU/ml  (Colony forming unit /milliliter) for liquids.

  CFU/g    (Colony forming unit /gram) for solids.

·         CFU can be calculate using miles and misra method, it is useful  to determine the microbiological       load.

\     The CFU/ ml can be calculated using the formula.
       
      CFU/mL = (no.of colonies x dilution factor) / volume of culture plate.

      For example:

        ·         suppose the plate of the 10⁶ dilution yielded a count of 130 colonies. Then, the number of      bacteria in 1 ml of the original sample can be calculated as follows:

·         Bacteria/ml = (130) x (10⁶) = 1.3 × 10^8 or 130,000,000.

CFU/mL Calculation Examples:

5 ml of Bacterial Culture is added to 45 ml of sterile diluent (0.9% saline). From this suspension, two serial, 1/100 dilutions are made, and 0.1 ml is plated onto Plate Count Agar from the last dilution. After incubation, 137 colonies are counted on the plate. Calculate CFU/mL of the original Sample?

First thing we need to know is the Dilution Factor, or how much the original sample is diluted:
here Initially 5mL in 45mL = Final Volume / Sample volume = 50/5 = 10.

Then two serial dilutions of 1/100.

Total Dilution Factor = 10 x 100 x100 = 10⁵

CFU/mL = cfu/ml = (no. of colonies x dilution factor) / volume of culture plate

= (137 x 10⁵)/0.1
=1.37 x 10⁸

So Total colony forming units = 1.37 x 10⁸ CFU/mL

Converting CFU/mL to Log value

For example,

Total colony forming units = 1.37 x 10⁸ CFU/mL and you want to convert it into Log value,

Just take Log(CFU/mL)

Here, log (1.37 x 10⁸ ) = 8.13924922.

Useful for expressing log reduction of microbes / biologic log reduction.