In microbiology, colony-forming unit (CFU) is an estimate of viable bacterial or fungal numbers. Unlike direct microscopic counts where all cells, dead and living, are counted, CFU estimates viable cells. The results are given as CFU/mL (colony-forming units per milliliter) for liquids and CFU/g (colony-forming units per gram) for solids.
Counting colonies is traditionally performed manually using a pencil and a click-counter. This is generally a straightforward task but can become very laborious and time consuming when many plates have to be enumerated. Alternatively, semi-automatic (software) and automatic (hardware + software) solutions can be used. Colonies can be enumerated from pictures of plates using software tools. The experimenters would generally take a picture of each plate they need to count and then analyze all the pictures (this can be done with a simple digital camera or even a webcam). Since it takes less than 10 seconds to take a single picture, as opposed to several minutes to count CFU manually, this approach generally saves a lot of time. In addition, it is more objective and allows the extraction of other variables such as the size and color of the colonies.