CFU Calculator
A CFU calculator helps estimate the number of colony-forming units (CFU) in a sample from microbiology culture data. It is commonly used to calculate microbial concentration from colony counts, dilution factors, and the volume of sample plated.
What Is CFU?
CFU stands for colony-forming units. It is a measurement used to estimate the number of viable microorganisms in a sample based on their ability to form colonies under specific culture conditions.
One CFU does not necessarily represent one individual microorganism because a colony can develop from a single cell or a group of cells.
CFU Formula
For a diluted sample, the concentration can commonly be calculated as:
CFU/mL = Number of colonies Γ· (Dilution factor Γ Volume plated in mL)
If the dilution is written as a fraction, such as 10β4, the dilution factor is its reciprocal, 104.
How to Calculate CFU/mL
Suppose a plate has 85 colonies, the sample dilution is 10β4, and 0.1 mL was plated.
The calculation is:
CFU/mL = 85 Γ· (10β4 Γ 0.1)
CFU/mL = 8.5 Γ 106
The estimated concentration is therefore 8.5 Γ 106 CFU/mL.
CFU Calculation Example
| Input | Value |
|---|---|
| Colony count | 120 |
| Sample dilution | 10β5 |
| Volume plated | 0.1 mL |
| Estimated concentration | 1.2 Γ 107 CFU/mL |
CFU From Multiple Plates
If several plates from the same dilution are counted, an average colony count can be used when appropriate. For example, if three plates contain 82, 86, and 84 colonies, the average is:
(82 + 86 + 84) Γ· 3 = 84 colonies
The average count can then be used with the dilution and plated volume to estimate the original concentration.
CFU/mL vs. CFU/g
CFU/mL is generally used for liquid samples, while CFU/g is commonly used for solid or semi-solid samples. The calculation must account for the original sample mass or volume and any dilution or preparation steps.
Why Is CFU Important?
CFU measurements provide an estimate of viable microbial concentration. They are used in microbiology research, food and water testing, pharmaceutical testing, environmental studies, and quality-control procedures.
Important Considerations
CFU results depend on the culture medium, incubation conditions, dilution accuracy, and the ability of microorganisms to grow under the selected conditions. Therefore, CFU is an estimate of viable, culturable microorganisms rather than necessarily a direct count of every cell present.
Common Mistakes to Avoid
Make sure the dilution is interpreted correctly. A plate from a 10β4 dilution represents a dilution factor of 104 when calculating the original concentration.
Also convert the plated volume to milliliters when calculating CFU/mL. Using the wrong volume or dilution factor can change the result by orders of magnitude.
Quick Summary
A CFU calculator estimates microbial concentration from colony counts, dilution, and plated volume. For liquid samples, a common formula is CFU/mL = colonies Γ· (dilution factor Γ volume plated in mL). The result represents an estimate of viable colony-forming units in the original sample.