A Most Probable Number (MPN) is an estimate of the mean density of viable microorganisms in a sample. A microbiologist prepares a step series of dilutions of a sample in tubes to a point where tubes will sometimes, but not always, contain viable microorganisms. Certain probability formulas (see references) are applied to the observed pattern of tubes containing viable microorganisms which consider the number of tubes and the dilution applied to the sample in each tube.
An MPN can be computed for any positive number of tubes at any positive number of dilutions, but often serial dilutions use three or more dilutions and a decimal series (the volume of inoculum used for each row is one tenth as much as the original sample as the previous row).
For more on the uses, applications, as well as the equations running it, you can read the FDA's page on it here.
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- FDA Bacteriological Analytical Manual, October 2010, Appendix 2, Most Probable Number from Serial Dilutions
- Russek, E. & Colwell, R. R. Computation of most probable numbers. Applied and Environmental Microbiology 45, 1646-1650 (1983)
- Briones, A. M. & Reichardt, W. Estimating Microbial Population Counts by 'Most Probable Number' Using Microsoft Excell. Journal of Microbiological Methods, 35, 157-161 (1999).