Colony forming unit (CFU)
The number of colony forming units (CFU) is the number of living microorganisms e.g. in water. For the determination, a quantity of water (e.g. 1ml) is incubated on a nutrient medium (Petri dish) and the microorganisms are stimulated to multiply at e.g. 30°C. The “units” (accumulations of microorganisms) counted after a defined time of incubation corresponds to the number of CFU. According to the German Drinking Water Ordinance, for example, 100 CFU per milliliter of drinking water are permissible under normal conditions. Exceptions to this are pathogens such as Legionella or E.Coli, the number of which must be zero.