colonyforming

Colony-forming describes the ability of microorganisms, particularly bacteria and fungi, to reproduce on solid growth media to form discrete colonies. In practice, the term is often tied to colony-forming units (CFU), a common metric used to estimate the number of viable, replicating cells in a sample. A CFU is defined as a colony that results from a single viable cell or from a minimal cluster capable of growth under the assay conditions; thus CFU counts reflect viable cells rather than all cells present.

Colony-forming assays typically begin with a sample that is serially diluted and plated onto solid agar. After incubation, individual colonies are counted on plates containing a suitable medium. The number of CFUs per unit volume or weight is calculated by multiplying the counted colonies by the dilution factor. Variants of the method include spread plate and pour plate techniques, and sometimes broth-to-plate transfers for more accurate estimation.

Applications of colony-forming counting include assessment of microbial load in clinical samples, foods and beverages, water, and environmental samples; evaluation of sterilization efficacy; and monitoring of fermentation processes. The method provides a practical measure of viable, culturable cells, though it does not detect cells that are viable but nonculturable under the chosen conditions, nor does it always distinguish species without further testing.

Interpretation must consider factors such as plating efficiency, clumping of cells, choice of media and incubation conditions, and potential growth inhibitors. Colony morphology and size can also give qualitative clues about identity and health of cultures.