CFU: Colony Forming Unit & Calculation

colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal cells. In direct microscopic counts where all cells, dead and living, are counted,but CFU measures only viable cells. For convenience the results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units per gram) for solids. CFU can be calculated using miles and misra method, it is useful to determine the microbiological load and magnitude of infection in blood  and other samples.

Example:
Calculate the number of bacteria (CFU) per milliliter or gram of sample by dividing the number of colonies by the dilution factor The number of colonies per ml reported should reflect the precision of the method and should not include more than two
significant figures.

The CFU/ml can be calculated using the formula:

cfu/ml = (no. of colonies x dilution factor) / volume of culture plate

For example, suppose the plate of the 10^6 dilution yielded a count of 130 colonies. Then, the number of bacteria in 1 ml of the original sample can be calculated as follows:

Bacteria/ml = (130) x (10^6) = 1.3 × 10^8 or 130,000,000.

Sources
http://en.wikipedia.org/wiki/Colony-forming_unit
Harley, Prescott, Laboratory Exercises in Microbiology, 5th Edition
Image Source: http://biology.clc.uc.edu/fankhauser/Labs/Microbiology/Yeast_Plate_Count/Yeast_Plate_Count.htm