What are intestinal enterococci?

Enterococci belong to the bacterial genus Enterococcus. They are round, Gram-positive bacteria, observed singly, in pairs, or in short chains. They are part of the normal intestinal flora of many humans and animals.

Classifications have evolved. These bacteria were formerly linked to group D streptococci and are still sometimes referred to as fecal streptococci, fecal enterococci, or intestinal enterococci. In documents regarding water, these terms may be interchangeable, but one must adhere to the exact terminology of the regulation or the method used.

Which species belong to the enterococci group?

The genus Enterococcus includes many species. The species Enterococcus faecalis and Enterococcus faecium are among those most frequently found in human and animal feces, wastewater, and certain contaminated water sources.

A routine regulatory analysis generally enumerates the intestinal enterococci group without identifying each species. Complementary typing may be requested for specific investigations, but it is not necessary to confirm a non-compliance in drinking water.

Why are enterococci used as indicators?

Directly searching for every possible pathogen in every sample would be technically complex, slow, and expensive. Authorities therefore use indicator bacteria to verify the sanitary integrity of the water.

• they are present in human and animal feces;
• they can signal a potential entry route for fecal contamination;
• they are detectable via standardized methods;
• they can complement the information provided by E. coli;
• they help assess the source, treatment, and distribution network.

Their presence does not automatically allow us to know the human or animal origin of the contamination. Nor does it allow us to identify a specific virus, parasite, or pathogenic bacterium.

What is the difference between intestinal enterococci and E. coli?

E. coli is generally considered the primary indicator of recent fecal contamination. Enterococci provide complementary information, particularly because they may be more resistant to certain environmental stresses and persist longer in some water sources.

A sample may be positive for enterococci and negative for E. coli. This does not allow the result to be ignored: both parameters have a quality limit of zero per 100 mL.

How do enterococci get into the water?

They can enter a source or a network when human or animal feces come into contact with the water.

• wastewater discharge or sewer overflow;
• leakage from a septic tank or non-collective sanitation system;
• runoff on pastures, farms, or contaminated soil;
• infiltration into an insufficiently protected well or spring;
• flooding of a catchment, reservoir, or wellhead;
• failure of filtration or disinfection;
• pipe rupture, pressure loss, or backflow;
• animal intrusion into a storage unit;
• contamination of the tap or equipment during sampling.

Enterococci can also be found in certain soils, sands, plants, or sediments. In drinking water, their presence nevertheless remains a signal serious enough to trigger a health investigation.

What situations favor contamination?

How long can enterococci persist?

Their survival varies according to temperature, exposure to sunlight, salinity, humidity, nutrients, particles, and the presence of other microorganisms. Health Canada reports persistence ranging from a few hours to several weeks in water, and sometimes several months in protective environments such as soil, sand, or plant matter.

In some groundwater, enterococci can persist longer than E. coli. This characteristic explains their value as a complementary indicator when contamination is intermittent or occurred a few days prior.

Do enterococci present in water directly cause disease?

In drinking water control, they are primarily sought as indicators. Their presence signals the possibility that fecal-borne pathogens may also be present.

Some species, notably E. faecalis and E. faecium, can cause opportunistic infections, especially in frail or hospitalized individuals. They can be involved in urinary, bloodstream, abdominal, or cardiac infections.

However, the detection of the enterococci group in water does not prove that the strain is virulent or responsible for an infection. Health Canada indicates that the consumption of treated drinking water has not been reported as an exposure route leading to these opportunistic infections.

What does antibiotic resistance in enterococci mean?

Certain strains of Enterococcus faecium or Enterococcus faecalis can be resistant to several antibiotics. Vancomycin-resistant enterococci are of particular concern in healthcare settings.

A regulatory drinking water analysis according to NF EN ISO 7899-2 does not automatically determine antibiotic resistance, resistance genes, or virulence. Specific analyses would be required for this.

What is the regulatory limit in drinking water?

French regulations and Directive (EU) 2020/2184 set a quality limit of 0 intestinal enterococci per 100 mL of water intended for human consumption.

Any result greater than zero in 100 mL of drinking water is non-compliant. The measures to be taken depend on the context, the number observed, other parameters, the installation, and the populations exposed.

Why should bathing water values not be applied to drinking water?

Enterococci are also used to assess the quality of bathing water. This framework accepts measurable concentrations and uses series of results to classify bathing areas.

These values are not applicable to drinking water. For drinking water, the French and European limit remains absence in 100 mL. Water that might be compliant for recreational use would therefore not necessarily be compliant for consumption.

How to collect a water sample to test for enterococci?

Improper sampling can artificially contaminate the sample or, conversely, reduce the observed concentration. The protocol must be defined with the laboratory according to the objective of the analysis.

1. use a sterile bottle provided by the laboratory;
2. choose a point representative of the issue under study;
3. remove faucet accessories when the protocol requires it;
4. clean or disinfect the point according to instructions;
5. let the water run for the requested duration;
6. do not touch the inside of the cap or the bottle;
7. never rinse a bottle containing a neutralizing agent;
8. keep the sample cold and transport it quickly.

For chlorinated water, the bottle generally contains sodium thiosulfate to neutralize the disinfectant during transport.

How are enterococci analyzed?

In France, the recognized method for water intended for human consumption is NF EN ISO 7899-2. It is based on membrane filtration, incubation on a selective medium, and a confirmation step.

Culture methods may fail to detect certain cells that have become viable but non-culturable. Conversely, a molecular method may detect genetic material without proving that a bacterium is capable of forming a colony.

How to interpret the result?

Result: absence in 100 mL

No intestinal enterococci were detected in the analyzed volume. The result is compliant for this parameter, but does not guarantee the absence of all pathogens or the absence of intermittent contamination.

Result: presence in 100 mL

The result is non-compliant for drinking water. Users must be protected, other microbiological parameters must be examined, and the source of the contamination must be investigated.

Positive enterococci and negative E. coli

This combination remains a non-compliance. It may be linked to different persistence, low or intermittent contamination, the type of resource, or the limits specific to each method.

What does a positive result in raw water or a private well mean?

A raw resource may contain microorganisms before treatment. Its quality helps define the necessary potabilization barriers. The methods used for surface and groundwater may differ.

In a private well used for drinking, the presence of enterococci indicates possible fecal contamination. Water should not be consumed until the source has been investigated, corrective measures have been taken, and new checks have confirmed compliance.

• inspect the well head, casing, and seals;
• check surface runoff and the slope of the ground;
• check proximity to sewage systems or livestock;
• look for the effects of heavy rain or flooding;
• correct the cause before any ad-hoc disinfection;
• perform a new analysis after intervention.

How does collective treatment control enterococci?

Treatment is based on a multi-barrier approach adapted to the quality of the resource.

1. Catchment protection: reduce fecal input.
2. Coagulation and clarification: remove carrier particles.
3. Filtration: reduce bacteria and turbidity.
4. Disinfection: inactivate remaining bacteria.
5. Protected storage: avoid recontamination.
6. Network monitoring: monitor disinfectant, pressure, and indicators.

Health Canada reports that conventional physical processes can produce reductions of around 1 to 4 logs for indicator bacteria under certain conditions, and that membranes can achieve higher reductions. These values must not be applied automatically to a particular installation without validation of the process.

Which disinfectants are effective?

Chlorine, chloramines, chlorine dioxide, ozone, and ultraviolet light can inactivate enterococci when the dose, contact time, and hydraulic conditions are adapted.

Disinfection does not replace the repair of a broken pipe, a flooded well, an open storage tank, or a permanent intrusion.

Which filters can retain enterococci?

Enterococci are micrometer-sized bacteria. Properly designed membranes can form a physical barrier, provided the device is intact and sealed.

Does boiling inactivate enterococci?

Yes. In the event of an official boil water notice, you must follow the authority's instructions exactly. In the absence of more precise instructions, the CDC generally recommends bringing clear water to a rolling boil for at least one minute, then letting it cool in a clean, covered container.

1. let cloudy water settle and pre-filter it;
2. bring the water to a rolling boil;
3. maintain boiling for the recommended duration;
4. let cool without adding unsafe ice;
5. store the water in a clean, protected container.

Boiling inactivates microorganisms but does not remove chemical contaminants and does not correct the cause of contamination.

How to evaluate a domestic device?

For water presenting a microbiological risk, one must examine the demonstrated performance of the device and not just its material.

• identify the exact model and the tested cartridge;
• check the microorganism used during the test;
• look for the reduction level and the duration of the protocol;
• check the flow rate and total volume treated;
• verify the seal and absolute pore size;
• follow cleaning and replacement rules;
• protect the tank and faucet against recontamination.

How to prevent contamination?

• protect catchments and well heads against runoff;
• maintain sewage systems;
• secure reservoirs, vents, and overflows;
• avoid cross-connections and backflow;
• disinfect and monitor networks after work;
• analyze wells after flooding or intrusion;
• maintain sufficient pressure in the network;
• maintain domestic devices according to their manual;
• do not consume uncontrolled river, rain, or spring water.

What to do if enterococci are detected?

1. consider the result as a microbiological non-compliance;
2. follow the instructions of the distributor, town hall, or health authority;
3. use alternative safe water or boil according to instructions;
4. do not prepare ice cubes, baby bottles, or food with the restricted water;
5. investigate the source within the resource, treatment, storage, and network;
6. check for E. coli, coliforms, and other useful parameters;
7. correct the cause and disinfect when necessary;
8. carry out control analyses before returning to normal usage.