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What's the Difference between MPN and CFU?

We get asked a lot of questions – as I am sure you do during the course of your day. “What is the detection limit of Colilert?” (1 organism/100mL) “Why does Legiolert only test for Legionella pneumophila?” (Because it causes 97% of Legionnaires’ disease cases.) “Where are your tests manufactured?”  (proudly developed and manufactured in Westbrook, Maine, USA.)

But the one question that we are most familiar with is ‘what’s the difference between the Most Probable Number (MPN) and a Colony Forming Unit (CFU)?’

The simple answer is that they are equivalent- one MPN is equal to one CFU. Both units measure the estimated number of bacteria in a water sample. Both are recognized by a variety of scientific and regulatory bodies worldwide including the US Environmental Protection Agency (EPA) and the International Standardization Organization (ISO). The difference is in how the measurement is obtained. The use of either MPN or CFU is based on the method used for the detection of bacteria and both are valid measurements for bacteria limits.

For CFUs, bacteria grow on a solid medium, like agar.  Afterward, colonies of bacteria are counted.  For an MPN measurement, samples grown in a liquid medium, like multiple tube fermentation and Colilert.  Positive wells/tubes are then counted and an MPN conversion table is used to generate a numeric result.  

In short, laboratories and agencies worldwide use both MPN and CFU interchangeably. 

Both measurements are well-established means to estimate the number of bacteria in a sample, and both carry a 95% confidence interval.  Membrane Filtration methods are reported in CFU, require multiple steps, and can take additional time for a confirmed result.  IDEXX tests are reported as MPN and provide faster results with no need for confirmation.  

We’ve put together a video to help understand the difference: