Lab 8: Most Probable Number Method (MPN)

Most Probable Number Method (MPN) What is the MPN method? How to determine the amount of bacteria from the MPN method? Objective

Most Probable Number Method (MPN)  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. MPN can be adjusted to provide a sensitive method to determine differences between two qualitative microbiological methods. As such, it can be used as a tool in validation of rapid microbiological methods and for growth promotion testing of broth media.

Most Probable Number Method (MPN)  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.  If, in the microbiologist's experience, the bacteria in the prepared sample in question can be found attached in chains that are not separated by the preparation and dilution, the MPN should be judged as an estimate of growth units (GUs) or colony-forming units (CFUs) instead of individual bacteria.

Most Probable Number Method (MPN)  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.  In order to obtain estimates over a broad range of possible concentrations, microbiologists use serial dilutions, incubating several tubes (or plates, etc.) at each dilution.

Confidence Intervals  The 95 percent confidence intervals in the tables have the following meaning.  Before the tubes are inoculated, the chance is at least 95 percent that the confidence interval associated with the eventual result will enclose the actual concentration.

Most Probable Number

 The basic concept for the MPN method is to dilute the sample to such a degree that inocula in the tubes will sometimes (but not always) contain viable organisms.  By replicates, and dilution series, this will result in a fairly accurate estimate of the most probable number of cells in the sample.  While this method is most commonly used in the personal products, medical device, and pharmaceutical QC microbiology labs for water testing, it has significant potential for other applications. SUMMARY

END OF LECTURE