Multiply the average number of colonies on an agar plate by the dilution factor to find the number of colony-forming units. Colony-forming units give indication of the number of culturable bacteria and fungi in an aqueous biological sample.
Starting from a liquid sample of known volume, add a known volume of sterile diluting agent. Next, divide the volume of the original liquid sample by the total volume of sample plus diluting agent to find the initial dilution. This initial dilution can be further diluted to even lower concentrations in cumulative steps. For example, by adding 1 milliliter of a sample of 1/10th initial dilution to 9 milliliters of sterile diluting agent, a 1/100 diluted sample can be prepared. To find the total dilution, multiply the fractions of all dilutions. The larger the number of dilutions, the smaller the amount of original sample taken will be.
The reciprocal of this total dilution is the dilution factor. To find this reciprocal, flip the fraction of the total dilution by dividing it by one. After cultivating a sample on an agar plate and counting the average number of colonies on the plate, the number of colony-forming units can be found by multiplying the number of colonies by the total dilution factor. The unit of colony-forming units is a number per unit volume or number per unit mass, such as CFU per milliliter or CFU per gram.