cfu

CFU stands for colony-forming unit, a unit used in microbiology to estimate the number of viable microorganisms in a sample. The concept relies on the ability of living cells to replicate and form visible colonies on solid growth media after incubation. A single microorganism or a cluster of several organisms can give rise to one colony, so a CFU count reflects viable units rather than individual cells.

Measurement is typically performed by plate counting. A serial dilution of the sample is prepared, and aliquots are inoculated onto agar plates by spread plating or pour plating. After incubation at an appropriate temperature and time, colonies are counted and reported as CFU per milliliter (CFU/mL) or CFU per gram (CFU/g), depending on the sample. For dilute samples, membrane filtration is also used, with the filter placed on an agar surface and incubated.

Interpretation and limitations: CFU estimates are influenced by the growth conditions, including medium composition, temperature, and incubation period. Some organisms enter a viable but non-culturable (VBNC) state and do not form colonies under standard conditions, leading to underestimation. Clumping of cells can result in fewer colonies than the actual viable cell count; conversely, multiple cells in a single colony can overgrow others and complicate enumeration. The method provides only an estimate of the viable, cultivable fraction of a population.

Applications: CFU counts are widely used in clinical microbiology to assess patient samples and infection risk, in food safety and beverage testing to evaluate contamination levels, in water quality monitoring, and in industrial settings such as fermentation and quality control. Standards bodies such as ISO, EPA, and FDA provide guidelines for CFU-based analyses.

Related concepts include plate count, viability assay, and viable cell count.