Glossary Item
A colony-forming unit (CFU) is a unit used in microbiology to estimate the number of viable microorganisms in a sample. It refers to a single cell or a cluster of cells capable of growing into a visible colony under specific conditions. Microorganisms often occur in chains or clumps, so counting colonies on an agar plate yields an estimate of living cells rather than an exact number of individual organisms. CFUs are typically expressed per millilitre or gram of sample.
Explanation
Microbiologists use CFU counts to quantify microorganisms in food, water, soil and clinical specimens. The method involves serially diluting a sample, spreading a known volume onto an agar plate and incubating the plate under conditions that support growth. After incubation, the number of colonies that appear on the plate is counted and used to calculate the concentration of viable cells in the original sample using the formula CFU equals the number of colonies divided by the product of the volume plated and the dilution factor. Only viable cells divide and form colonies, so CFU counts exclude dead cells and provide a measure of live organisms. The accuracy of the method depends on obtaining plates with a countable number of colonies, usually between 30 and 300, achieved through serial dilutions. CFU counts have broad use: assessing microbial contamination in drinking water and food products, evaluating the effectiveness of disinfectants and antibiotics, determining the potency of probiotic supplements, and measuring cell proliferation in cell culture experiments. These counts are also used in clinical diagnostics to gauge the severity of infections and guide treatment decisions. Some organisms grow in chains or clusters, which can lead to an underestimate of the actual number of viable cells.
Key Applications
CFU measurements are integral to water quality monitoring, where coliform counts indicate faecal contamination and help authorities assess the safety of drinking and recreational waters. In food safety testing, CFU counts reveal the microbial load on raw vegetables, meat and dairy products and are used to verify that processing and storage practices limit bacterial growth. Researchers rely on CFU counts to compare the growth of mutant and wild‑type strains, evaluate the impact of antimicrobial treatments, and quantify the effectiveness of sanitation protocols. Manufacturers of probiotic supplements list CFU numbers to convey how many live bacteria are present in each dose. These diverse applications show how a simple colony count can inform public health decisions, product development and basic research.
Counting colonies on agar plates is an inexpensive and accessible technique that remains central to microbiology. CFU measurements give laboratories an estimate of the number of live microorganisms without specialized equipment. A clear grasp of what a CFU represents helps practitioners interpret test results and recognize limitations of the method, such as undercounting when cells clump together.
Related Terms: colony, serial dilution, agar plate, viable count, culture