Colony forming unit

Colony forming unit (CFU) is a measurement used to estimate the number of viable bacteria or fungal cells in a sample. A cell is only considered viable if it is able to multiply via binary fission under the controlled conditions.

The colony forming units can be calculated by spreading a specific volume of the microbial culture onto an agar plate. After incubating the agar plate the number of colonies is proportional to the number of viable colonies in the tube.

For example:

If 0.1 mL of a bacterial culture results in 25 colonies. The colony forming units per milliliter can be calculated as follows:

cfu/mL = colonies/volume spreadcfu per milliliters is equal to colonies divided by volume spread

25 colonies / 0.1 mL = 250 cfu/mL25 colonies divided by 0.1 milliliters is equal to 250 cfu per milliliters

As many bacteria cultures contain many viable cells it is not possible to count the colonies directly. In this case, you need to perform a serial dilution in order to decrease the number of cells that you use for spreading. In order to calculate the cfu/mL of a diluted sample, you need to multiply the number of colonies with the dilution factor.

The number of bacteria in a sample can also be quantified with other techniques such as examination with a microscope. These alternative methods require significant growth and also don't exclude dead or non-viable cells.