Colony Forming Unit (CFU)
The amount of bacteria in a solid medium, for example, food is often expressed as colony forming units per gram, (CFU/g), whereas the number of bacteria in a liquid sample is expressed as colony forming units per ml (CFU/ml). This value can be determined by first making a tenfold dilution series of the sample. Then, a known volume (usually 1 ml or 0.1 ml) is taken from each dilution and is applied to appropriate agar plates. After incubation, the number of colonies is counted on the agar plate with 15-150 colonies, which is considered to give the most accurate result. When calculating the actual number of bacteria in the sample, the dilution factor also has to be considered.
In assessing urine samples, the number of bacteria per ml (CFU/ml) is of great importance. Urine is collected with a loop of a known volume (1 µl) and distributed on the agar plate to facilitate counting of the number of colonies. The amount of bacteria in urine is usually classified in the following way.
Note that this method to determine the amout of bacteria in urine, only gives a rough estimate of the actual number.
Bacteriuria is defined as the presence of bacteria in urine in almost pure culture with a concentration of more than 1 000 CFU/ml from urine sampled by cystocentesis and more than 100 000 CFU/ml from spontaneous urination. A patient is said to have urinary tract infection (UTI) when there is bacteriuria and clinical symptoms of infection.
The reason for using the term CFU is that it is not certain that each colony originates from a single bacterial cell, because some bacterial species easily form aggregates in suspension cultures. Only living (or rather culturable) bacteria will give rise to colonies.