Most Probable Number Method (MPN)

What is the MPN method?

How to determine the amount of bacteria from the MPN method?

The most probable number (MPN) method is familiar to quality control (QC) microbiologists as part of the microbial limits tests.

Its usefulness goes far beyond this one test, however.

The theory behind the MPN method is central to the commonly used D-value determination by fraction-negative method, and a variant of this method has been suggested for trending of environmental monitoring data from the aseptic core.

The most probable number (MPN) is particularly useful for low concentrations of organisms (<100/g), especially in milk and water, and for those foods whose particulate matter may interfere with accurate colony counts.

Only viable organisms are enumerated by the MPN determination.

For simplicity, however, here we will speak of these GUs or CFUs as individual bacteria.

The following assumptions are necessary to support the MPN method.

The sample is prepared in such a way that the bacteria are distributed randomly within it.

The bacteria are separate, not clustered together, and they do not repel each other.

The growth medium and conditions of  incubation have been chosen so that every inoculum that contains even one viable organism will produce detectable growth.

The essence of the MPN method is the dilution of a sample to such a degree that inoculate will sometimes but not always contain viable organisms.

The “outcome”, i.e., the numbers of inoculate producing growth at each dilution, will imply an estimate of the original, undiluted concentration of bacteria in the sample.

Confidence Intervals

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En Muhtemel Sayı (EMS) Yöntemi : Yöntemin prensibi ardışık 3 seyreltiden sıvı besiyerlerine ekim yapıp inkübasyon sonunda gelişme olanları pozitif olarak değerlendirmek ve istatistik yöntemlerle elde edilmiş tablolardan yararlanılarak örnekteki sayıyı hesaplamaktır. Yöntemin “En Muhtemel Sayı (Most Probable Number ; MPN) olarak adlandırılma nedeni yukarıda da belirtildiği gibi örnekteki mikroorganizma sayısının istatistik şekilde elde edilmiş tablolardan yararlanılarak hesaplanmasıdır.