Colony-forming unit[cfu] : Microbiological unit estimating viable microbial cells in a sample

colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal cells. In direct microscopic counts where all cells, dead and living, are counted. 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. The process involves culturing the microbes on agar plates, often through serial dilution of the original sample to ensure accurate counting. Each visible colony on an agar plate can be multiplied by the dilution factor to determine the CFU/ml value. It’s important to note that CFU counts may underestimate the actual number of viable cells due to the possibility of colonies originating from cell clumps rather than individual cells. 

CFU/mL Calculation Example:
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.

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?

Answer:

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

=1.37*10^8

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.