Waksman Foundation of Microbiology's:
Summer Science Academy Experiment: Estimating the Number of Bacteria on a Solid Surface
In microbiology, colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal numbers. Unlike direct microscopic counts where all cells, dead and living, are counted, CFU measures 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.
The theory behind the technique of CFU is to establish that a single bacterium can grow and become a colony, via binary fission. These colonies are clearly different from each other, both microscopically and macroscopically. However, some bacteria do not separate completely during the sample preparation process (Staphylococcus, Streptococcus) and the results of the count will be below the number of individual cells using direct methods.
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