Enterococci Detection Offers a Robust Method for Water Quality Test

That's why enterococci detection is playing an increasingly important role in monitoring recreational waters and wastewater. Enterococci offer additional insight into contamination risks and public health concerns because they persist longer than many other microorganisms, including E. coli.

During the recent IDEXX Water Academy webinar "Enterococci Detection: Effective Testing Methods for Water Quality," Shawn Dubois, senior scientist for IDEXX Water, spoke at length about enterococci.

"Enterococci are just another one of those pieces that we can use to assess water quality," Dubois said. "Their ability to survive in conditions that other bacteria can't makes them a really useful indicator [of fecal contamination]."

Below, we'll explore the enterococci detection processes, including the environments in which they flourish, what their presence signifies, and special considerations for challenging environments, such as marine waters and high-load wastewater systems.

Enterococci Detection Can Indicate Fecal Contamination

Although the presence of enterococci isn't inherently harmful, their detection strongly signals the potential presence of other disease-causing organisms. Their resilience in adverse environments sets them apart from more fragile indicators, like fecal coliforms or E. coli.

Enterococci can tolerate harsh conditions, including high salinity, exposure to disinfectants like sodium azide, and fluctuations in pH and temperature. This hardiness allows them to persist in environments where other microbes might quickly die off, providing a more reliable signal of potential fecal contamination.

They can even survive in the presence of bile salts, which normally inhibit bacterial growth. This durability makes enterococci especially useful in monitoring beach waters and effluent from wastewater treatment facilities, where environmental conditions can be daunting and require special considerations.

Enterococci Simplify Tests in Key Environments

Enterococci contaminate lakes, rivers, coastal beaches, untreated and treated wastewater, and groundwater influenced by surface runoff or septic systems. In rare cases, they may be present in drinking water systems, particularly in countries with evolving water infrastructure.

In North America, testing for enterococci is typically required in recreational water monitoring programs and wastewater compliance testing. Elevated levels may indicate recent fecal contamination and can prompt advisories, increased surveillance, or intervention by public health authorities. In Europe, regulatory shifts are elevating the role of enterococci in drinking water monitoring, particularly under the revised European Drinking Water Directive.

Utilities and labs that test for enterococci gain an essential early warning tool that fills in the gaps left by existing microbial testing strategies.

"If we're finding large quantities of enterococci, it's a good indication that there has been some kind of fecal contamination," Dubois explained. Their presence serves as a proxy for other pathogens that may be difficult or time-consuming to detect.

Defined Substrate Technology and Membrane Filtration Can Detect Enterococci

Generally speaking, there are two standard methods to test for enterococci in water: defined substrate technology (DST) and membrane filtration (MF). Each method has its strengths and appropriate applications, depending on the lab's workflow, available equipment, and testing volume.

Membrane Filtration Testing

MF is a more traditional approach. This test involves passing the sample through a fine filter, then placing the membrane on a selective growth medium. After incubation, technicians count colonies with specific colors and shapes, such as a blue halo.

While effective, this method is labor-intensive and may require further testing for true confirmation.

Defined Substrate Technology Testing

DST, which the IDEXX Enterolert implements, is a time-saving option. This testing protocol utilizes a liquid reagent that combines a nutrient and an indicator. If enterococci are present in the sample, they express an enzyme that cleaves the compound. Fluorescence or color confirms their presence. This method allows results to be read in as little as 24 hours without the need for further confirmation.

The IDEXX Enterolert-DW test for enterococci recently achieved a milestone by being officially published as the International Organization for Standardization (ISO) Standard 7899-3, which specifies the Most Probable Number (MPN) method for detecting and counting intestinal enterococci. This recognition validates Enterolert as a globally accepted method for detecting enterococci in both drinking and bathing water, providing water utilities and labs with an internationally recognized tool.

Dubois highlighted that both methods produce estimates of bacterial populations, but DST offers notable advantages in terms of efficiency and ease of use. DST's simplicity and accuracy make it a preferred method for many labs, especially those working with time-sensitive public health concerns.

"If you can make Kool-Aid, you can run this test," Dubois said during the webinar.

Samples That Require Special Consideration

When monitoring marine waters or high-load wastewater samples, lab teams must adjust methods to ensure accuracy. Due to the elevated salinity in coastal water and potential interference from other compounds in wastewater, IDEXX recommends diluting such samples 1:10 using sterile diluent.

Once you dilute the sample, you can conduct DST as usual. After incubation and result reading, you multiply the detected Most Probable Number (MPN) value by the dilution factor to yield the final result. For example, if a diluted sample yields 22.2 MPN, the final result would be 222 MPN/100 mL. The same scaling applies to confidence intervals.

The IDEXX Quanti-Tray/2000 format, with a higher counting range, enables the simultaneous processing of multiple samples without the need for dilution. Still, for marine or known high-count samples, it's considered best practice to dilute samples before testing.

Visual interpretation also plays a role. DuBois advised that even faint blue fluorescence counts as a positive.

"Some wells glow a brilliant white-blue, while others may have just a hint of grayish-blue," Dubois explained. "If there's blue, it's a positive."

To help interpret borderline cases, labs can use rubber tray inserts to isolate individual wells and reduce interference from adjacent bright positives.

Enterococci Detection Gives Labs a Robust Monitoring Tool

Enterococci testing provides a robust and reliable method for detecting fecal contamination in various water environments. As international drinking water standards continue to evolve, regulators increasingly consider enterococci, underscoring their growing importance in global public health.

With DST, labs can obtain rapid, accurate results with minimal equipment and effort. By integrating enterococci testing into routine monitoring—especially in saline or high-load samples—utilities and labs can better safeguard water quality and protect communities.

"[DST testing for enterococci] is one more tool we can use to make sure people are safe," Dubois said.