Most Probable Number (MPN)

Definition - What does Most Probable Number (MPN) mean?

The most probable number (MPN) is the number of organisms that are most likely to have produced laboratory results in a particular test. The MPN method is used to quantify the concentration of the viable microorganisms in a sample and involves inoculating decimal dilutions into tubes of a broth medium, observing results and using a standard MPN table.

The MPN method is used in a wide range of industries, including oil pipelines, where it used to estimate the concentration of biocorrosion organisms.

Corrosionpedia explains Most Probable Number (MPN)

The MPN technique involves using a decimal (ten-fold) dilution series of a sample and then inoculating one ml of each dilution into a separate tube containing a broth medium. After incubation, the broth tubes are observed for the presence or absence of growth. Any inocula with at least one organism will have a visible growth in the corresponding tube. A dilution without growth is assumed not to have any organisms.

In order to get a more accurate number, more than one broth tube is inoculated from each dilution. A typical test uses a minimum of three dilutions and three, five or ten tubes per dilution. The pattern of positive and negative tubes is then noted after the incubation and results checked against a standardized MPN table to determine the most probable number of microorganisms per unit volume of the original sample.

Advantages of the MPN technique include:

  • Ease of interpretation, either by observation or gas emission
  • Sample toxins are diluted
  • Effective method of analyzing several samples such as sediments, sludge, mud, etc.

Disadvantages include:

  • It takes a long time to get the results
  • Results are not very accurate
  • Requires more hardware (glassware) and media
  • Probability of false positives

The most probable number is used in the pharmaceutical industry, corrosion studies and water testing to monitor and develop control and preventive measures for the environment, steel, water etc. Typical applications are:

  • Detection and enumeration of bacteria in food and water to prevent infections and diseases
  • Analyzing the thiosulfate-reducing bacteria that contribute to biocorrosion in oil pipelines
  • Enumerating the microbial bacterial populations in environments where steel may suffer from microbiologically influenced corrosion (MIC)
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