What is colony-forming unit (CFU)?

• A. A unit used to estimate the number of viable bacteria or fungal cells in a sample
• B. A measure of total cells including dead cells
• C. A type of staining
• D. A method for measuring toxins

Answer: (A)

CFU, or colony-forming unit, is a unit used to estimate the number of viable bacteria or fungal cells in a sample. It reflects cells that are capable of growing into a visible colony on an appropriate culture medium. Because only living, reproducing cells form colonies, CFU counts exclude dead cells. In practice, a sample is serially diluted and plated; after incubation, the number of colonies is counted and multiplied by the dilution factor to give CFU per unit volume (for example, per mL). Remember, this is an estimate and can be influenced by factors like cell clumping or suboptimal growth conditions. It is not a measure of total cells, a staining method, or a toxin measurement.