This is the pillar article of the TAAG Learning Center on interpreting results. If you need the broader picture first, review what microbiological food analysis is.
What is a CFU (Colony-Forming Unit)?
CFU stands for colony-forming unit. It is the unit of measurement used in microbiology to estimate how many viable microorganisms are in a sample, based on the number of visible colonies that grow on a culture medium after incubation.
A CFU is not necessarily equivalent to a single cell: it can originate from an individual cell, a chain of cells or a cluster (for example, a clump of Staphylococcus) that, unable to separate during plating, gives rise to a single visible colony. That is why the result is reported as "colony-forming units" and not as "number of bacteria".
Why use CFU and not "number of bacteria"?
Because traditional microbiological analysis does not count cells one by one: it counts colonies visible to the naked eye after each grew from a point of origin on the plate. It is an indirect method, but internationally standardized, which allows results to be compared between different laboratories as long as they follow the same plating and incubation protocol.
How is the count in CFU/g, CFU/mL or CFU/cm² calculated?
The plate count formula
The basic calculation is: CFU/g (or mL) = number of colonies counted × (1 ÷ dilution) ÷ volume plated. For example, if 45 colonies were counted on a plate seeded with 1 mL of a 10⁻³ dilution, the result is 45 × 1,000 = 45,000 CFU/mL.
Serial dilutions and their role
Since many samples have too many microorganisms to count directly (the colonies would overlap), serial dilutions are prepared (1:10, 1:100, 1:1,000...) and several of them are plated. The plate falling within a countable range — normally between 25 and 250 colonies — is chosen for the calculation to minimize statistical error.
Surfaces: CFU/cm²
In the case of microbiological surface sampling, the result is normalized by the sampled area (for example, 100 cm²), which allows points of different sizes within the same plant to be compared.
Which CFU limits are acceptable?
There is no universal number: the acceptable limit depends on the microorganism analyzed, the matrix (food, water, surface), the applicable local regulations and customer specifications. The same aerobic mesophile count can be normal in a fresh vegetable and excessive in a pasteurized product. For pathogens such as Salmonella, many schemes directly require "absence" in the sample size analyzed, rather than a numerical CFU limit.
CFU vs. molecular methods (PCR/RNA): what does each measure?
The CFU count measures exclusively microorganisms capable of growing and forming a visible colony under the conditions of the culture used — that is, viable and culturable cells. Molecular methods such as PCR detect genetic material (DNA or RNA), which can include non-culturable cells or, in the case of DNA-based PCR, genetic remains of already inactive cells. This difference explains why a positive PCR result does not always match a positive CFU count exactly, and why both approaches are considered complementary rather than interchangeable.
Common mistakes when interpreting CFU
- Comparing the CFU/g of one matrix with the regulatory limit of a different matrix.
- Ignoring the countable range of the plate (for example, accepting a count from plates with fewer than 25 or more than 250 colonies without adjusting the result).
- Interpreting "0 CFU detected" as "total absence guaranteed", without considering the detection limit of the method.
- Not recording the dilution used, which makes it impossible to audit or repeat the calculation later.
Conclusion
CFU remain the common language of industrial microbiology: a standardized count, comparable between laboratories, that supports decisions on product release, cleaning or recall. Understanding how they are calculated, which limits apply according to the matrix and how they differ from molecular methods is what turns a number into a well-founded quality decision.
About TAAG
Discover how TAAG combines traditional CFU counts with AiGOR™ molecular detection to give a more complete reading of microbiological risk.
Frequently asked questions
Does a result of "0 CFU" mean there are no microorganisms in the sample?
Not necessarily. It means no colonies were detected above the detection limit of the method and the volume plated; microorganisms present at very low concentrations might not appear.
Why can two laboratories report different CFU for the same sample?
Differences in the culture medium, incubation temperature and time, plating technique or the dilution range chosen can generate variations, even when following the same reference standard.
Do CFU apply equally to bacteria, fungi and yeasts?
The concept is the same (visible colonies after incubation), but culture media, times and temperatures vary according to the microbial group analyzed.