Most Probable Number

The Most Probable Number (MPN) test is a lab method used to estimate the number of enterococci in a water sample. The sample is divided into multiple tubes or wells containing a special growth medium that changes colour or glows if enterococci are present. The tubes are incubated for 24 to 48 hours. After incubation, technicians count how many tubes show a positive result. Using a statistical MPN table, they calculate the most probable number of enterococci in the original sample, reported as MPN per 100 millilitres. This method is useful for testing low bacteria levels.