In microbiology, colony-forming unit (CFU) is a measure of viable bacterial or fungal numbers. Unlike direct microscopic counts where all cells, dead and living, are counted, CFU measures 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.
The theory behind the technique of CFU is to establish that a single bacterium can grow and become a colony, via binary fission. These colonies are clearly different from each other, both microscopically and macroscopically. However, some bacteria do not separate completely during the sample preparation process (Staphylococcus, Streptococcus) and the results of the count will be below the number of individual cells using direct methods.
Techniques such as the Miles and Misra Methods allows the determination of the number of CFU per mL in the sample, and thus the microbiological load and the magnitude of the infection in humans or animals, such as in blood cultures or urine cultures. Also, it may measure the degree of contamination in samples of water, vegetables, soil or fruits, and in industrial products and equipment.
Stem cells may also be measured in CFUs. For example, irradiated mice can have their immune systems reconstituted by the injection of bone marrow cells from a non-irradiated animal. The injected cells form colonies in the spleen, and each colony represents the progeny of a single pluripotent stem cell. Therefore, operationally, the number of colony-forming units is a measure of the number of stem cells. Colony-forming units are also very useful in industrial processes for producing biofertilizers.