The most probable number method, otherwise known as the method of
Poisson zeroes, is a method of getting quantitative data on concentrations of discrete items from positive/negative (incidence) data.
There are many discrete entities that are easily detected but difficult to count. Any sort of amplification reaction or
catalysis
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
reaction obliterates easy quantification but allows presence to be detected very sensitively. Common examples include
microorganism growth,
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
action, or catalytic chemistry. The MPN method involves taking the original solution or sample, and subdividing it by orders of magnitude (frequently 10× or 2×), and assessing presence/absence in multiple subdivisions.
The degree of dilution at which absence begins to appear indicates that the items have been diluted so much that there are many subsamples in which none appear. A suite of replicates at any given concentration allow finer resolution, to use the number of positive and negative samples to estimate the original concentration within the appropriate order of magnitude.
In microbiology, the cultures are incubated and assessed by eye, bypassing tedious colony counting or expensive and tedious microscopic counts.
Presumptive, confirmative and completed tests are a part of MPN.
In molecular biology, a common application involves
DNA templates diluted into
polymerase chain reaction
The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...
s (PCR). Reactions only proceed when a template is present, allowing for a form of quantitative PCR, to assess the original concentration of template molecules. Another application involves diluting enzyme stocks into solution containing a chromogenic substrate, or diluting antigens into solutions for
ELISA
The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...
(Enzyme-Linked ImmunoSorbent Assay) or some other antibody cascade detection reaction, to measure the original concentration of the enzyme or antigen.
The major weakness of MPN methods is the need for large numbers of replicates at the appropriate dilution to narrow the confidence intervals. However, it is a very important method for counts when the appropriate order of magnitude is unknown ''a priori'' and sampling is necessarily destructive.
See also
*
Dilution assay
External links
*A downloadable MPN calculator to take your data and get estimates
A five-replicate MPN tableInformation on the MPN method and ballast water treatment
References
{{reflist
*Oblinger, J.L., and J. A. Koburger, J.A. (1975) "Understanding and Teaching the Most Probable Number Technique." J. Milk Food Technol. 38(9), 540–545.
Quantitative research
Laboratory techniques
Biostatistics