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Monday, November 21, 2011

CFU: Colony Forming Unit & Calculation

colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal cells. In direct microscopic counts (cell counting using haemocytometer) where all cells, dead and living, are counted,but CFU measures only viable cells. For convenience the results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units per gram) for solids. CFU can be calculated using miles and misra method, it is useful to determine the microbiological load and magnitude of infection in blood  and other samples.


Calculate the number of bacteria (CFU) per milliliter or gram of sample by dividing the number of colonies by the dilution factor The number of colonies per ml reported should reflect the precision of the method and should not include more than two significant figures.

Stock/ml dilutions
Serial Dilution of Bacterial Culture 

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 10^6 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 (10^6) = 1.3 × 10^8 or 130,000,000.

CFU/mL Practice Problems - CFU/mL Calculation Examples

Problem 1:
Five ml of Bacterial Culture is added to 45 ml of sterile diluent. 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 * 100 *100 = 10^5

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

= (137 * 10^5)/0.1

So Total colony forming units = 1.37*10^8 CFU/mL

Converting CFU/mL to Log value

For example,

Total colony forming units = 1.37*10^8 CFU/mL and you want to convert it into Log value,

Just take Log(CFU/mL)

Here log (1.37*10^8) = 8.13924922.

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

Laboratory Exercises in Microbiology