MPN Versus CFU: Understanding Two Key Measures in Water Quality Testing

MPN is a statistical estimate based on detection across small, replicate portions of the original sample. CFU is a direct count of colonies grown on solid media. These MPN versus CFU differences affect test sensitivity as well as the equipment and labor required. The words to remember are "different methods, equivalent results."

Both techniques are well-established and included in ISO Standard 6107:2021 as acceptable units for estimates of bacteria numbers. In all cases, 1 MPN equals 1 CFU, and each method has a 95% confidence interval. Luckily for labs aiming to stay compliant, pass an audit, and deliver solid results, both CFU and MPN measurements are accepted by regulatory agencies worldwide.

Let's take a closer look at how each method works, the contexts in which they're used, and what you should consider when choosing between them.

What Is MPN?

MPN is a statistical approach to estimating the number of microorganisms in a sample. It's most commonly used when working with liquid media. The method assumes the bacteria are evenly distributed across the liquid and separated randomly into distinct wells or tubes.

First, a reagent is added to the water sample. A technician using an IDEXX test would seal the sample into a Quanti-Tray and then incubate it. After a specified incubation period, analysts examine each well or tube for a positive (growth) or negative (no growth) result, usually indicated by a color change or fluorescence. They can then interpret results using a probability table or software algorithm to estimate the most likely number of organisms in the original sample.

One of the key advantages of MPN is its ability to detect and quantify bacteria even at low concentrations. It's particularly useful for regulatory compliance, as seen in tests like IDEXX's Colilert® and Enterolert®, which are approved by the Environmental Protection Agency (EPA) under the Total Coliform Rule and Recreational Water Quality Criteria, respectively.

What Is CFU?

CFU represents a more traditional approach to counting bacteria. CFU is an estimate based on the number of colonies that grow on a solid medium—usually an agar plate—after incubation. This method assumes that each viable bacterium in the sample will grow into a distinct colony.

The process starts with membrane filtration. Smaller particles, like viruses and water molecules, pass through the membrane, while larger particles, like bacteria, remain on the membrane itself. The filter with the bacteria on it is then placed on a solid medium. The medium contains a food source that supports the bacteria while it replicates and forms colonies. An analyst can count the number of colonies to estimate how many bacteria are in the sample.

For moderate to high bacterial loads, CFU provides a direct, visual count of colonies, which many find intuitive and reliable. However, analysts may have difficulty distinguishing between different species or types of microorganisms without further testing. The method becomes less practical at low concentrations due to the large volumes of water that need to be filtered and plated for testing.

Comparing MPN versus CFU

The choice between MPN and CFU depends on the sample type, expected bacterial concentration, regulatory context, and available lab resources. Here are some key differences in performance characteristics:

• Detection sensitivity: MPN is generally more straightforward for detecting low concentrations of bacteria, making it ideal for potable water testing where limits are strict.
• Time and labor: MPN-based tests are often faster to set up and require less manual interpretation. Many MPN tests come in pre-dosed formats with color indicators for automation or quick reading.
• Precision: CFU counts are considered more precise at higher concentrations because they involve direct counting rather than inference using statistics.
• Regulatory acceptance: Certain regulations require or prefer one method over the other. In the U.S., the EPA approves MPN methods, like IDEXX Colilert, under specific rules, while CFU methods are still common in international and legacy applications.
• Matrix compatibility: MPN formats may be more suitable for turbid or particulate-laden samples, where solids could interfere with plating efficiency for CFU tests.

Making the Choice of MPN versus CFU

MPN and CFU both serve the same goal: quantifying microbial contamination. They differ, however, in approach and applicability. MPN offers speed, ease, and low-level detection well-suited to drinking and recreational water programs, while CFU delivers visual confirmation better for high-load or research samples.

Understanding these differences gives utility and water quality testing labs the foundation to select the most appropriate test method for their needs, ensuring accurate results, regulatory compliance, and safer water for every use.