Colony-forming unit
- Abbreviation
- CFU
- Also known as
- CFU, CFU/g, CFU/mL
A colony-forming unit (CFU) is a measure of the number of viable microorganisms in a sample, based on how many visible colonies grow when the sample is spread on a nutrient medium. Microbial product labels use it to state how many live organisms they contain.
What is a colony-forming unit?
Microorganisms are too small to count individually by eye. The standard way to count live bacteria or fungal spores is to dilute a sample, spread it on a plate of nutrient agar, and let it incubate. Each viable cell, or small clump of cells, multiplies into a visible colony. Counting the colonies and multiplying by the dilution gives the number of colony-forming units in the original sample.
Results are expressed as CFU per gram, per milliliter, or, for seed treatments, per seed.
Why “colony-forming unit” and not “cell”
The count reflects colonies, not individual cells. Two or more cells stuck together form one colony, and organisms that are alive but cannot grow on the chosen medium are not counted. CFU is therefore a measure of viable, culturable organisms, and it can differ from counts made by other methods, such as microscopy or DNA-based techniques.
CFU on product labels
For microbial inoculants and other biological products, CFU tells a user how many live organisms the product delivers. Labels typically state:
- The organism, often down to the strain.
- A guaranteed minimum CFU per unit of product.
- An expiration date, because viable counts decline over time.
A product’s effectiveness depends on delivering enough live organisms, so storage conditions, shelf life and handling that affect CFU also affect results in the field.
Frequently asked questions
Not necessarily. There must be enough organisms to establish, but strain quality, how well they survive to reach the plant, and field conditions matter as much as the number.
Microorganisms die during storage, faster at high temperatures and when dried or exposed to incompatible chemicals. This is why products carry expiration dates and storage instructions.
Sources
- Microbiology laboratory methods for plate counts, American Society for Microbiology
- Labeling of microbial agricultural products, Association of American Plant Food Control Officials (AAPFCO)