The difference between CFU and a bacterium cell

In microbiology, colony-forming unit (CFU) is a measure of viable bacterial or fungal numbers. Unlike in direct microscopic counts where all cells, dead and living, are counted, CFU measures viable cells. By convenience the results are given as CFU/mL, colony-forming units per milliliter.

The theory behind the technique of CFU establish that a single bacterium can grow and become a colony, via binary fission. These colonies are clearly different between each other, both microscopical and macroscopical. 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.

A COLONY is a cluster of microorganisms growing on the surface of or within a solid medium; usually cultured from a single cell.

Each bacterial CELL arises either by division of a preexisting cell with similar characteristics (i.e., binary fission), or through a combination of elements from two such cells in a sexual process.

Statistical significance, when applied to incubated plates, depends upon the research hypothesis, the power of the test (i.e., the probability of a Type II error (i.e., the likelihood of failing to reject the null hypothesis when it is not true)), the difference between the number of colonies observed under the experimental and control conditions, and the accuracy of the counting methodology.

Source(s):

http://en.wikipedia.org/wiki/Colony-form…
http://www.answers.com/topic/colony-word…
http://en.wikipedia.org/wiki/Statistical…
http://en.wikipedia.org/wiki/Type_I_and…