What is Escherichia coli?

Escherichia coli is a Gram-negative bacterium belonging to the Enterobacterales family. It lives primarily in the digestive tracts of humans and many animals. Its abundance in fecal matter and its relatively accessible detection explain its use as a microbiological indicator of water quality.

The presence of E. coli in a drinking water sample generally indicates the recent arrival of human or animal fecal matter, a treatment failure, an intrusion into the distribution network, or contamination during sampling.

Why is E. coli used as an indicator of fecal contamination?

A microbiological indicator is not necessarily the agent responsible for a disease. It serves to reveal a situation that could favor the presence of pathogens. E. coli is considered one of the most appropriate indicators of fecal contamination in drinking water.

• it is abundant in human and animal fecal matter;
• it is normally rare in water not contaminated by fecal matter;
• it can be tested for using standardized methods;
• its detection allows for the rapid identification of a sanitary failure;
• it triggers an investigation into the origin of the contamination.

However, E. coli is not a perfect indicator. Its survival and sensitivity to treatment differ from those of more resistant viruses, protozoa, or bacteria. Therefore, a negative spot result does not rule out all pathogens in every situation.

Are all E. coli strains dangerous?

No. Most E. coli strains live normally in the intestine without causing illness. Some, however, possess virulence factors that allow them to cause diarrhea, urinary tract infections, sepsis, meningitis, or other infections.

STECs are also called VTECs. The term EHEC is often used to refer to STEC strains capable of causing severe clinical forms. Serotype O157:H7 is well-known, but many non-O157 strains can also cause disease.

How does E. coli get into water?

Contamination results from water coming into contact with human or animal fecal matter. It can occur at the source, during treatment, in storage, in the distribution network, or at the point of use.

• wastewater discharge or sewage system overflow;
• septic tank leakage or malfunction of an individual sewage system;
• agricultural runoff or the presence of animals near a water intake;
• flooding of a well or reservoir;
• failure of a filtration or disinfection stage;
• pipe rupture, pressure drop, or backflow;
• poorly closed reservoir, unprotected vent, or animal intrusion;
• faucet contamination or error during sampling.

Rainwater is not sterile. After running off roofs, soil, or soiled surfaces, it can transport E. coli and other microorganisms.

Which situations increase the risk of contamination?

How long can E. coli survive in water?

Survival depends on temperature, light, presence of nutrients, salinity, pH, microbial competition, and time. Bacteria can persist longer in cool, murky, or nutrient-rich water than in water exposed to sunlight and low in nutrients.

In contaminated water and soil, some populations can persist long enough to be transported to a water source. However, a detection cannot precisely date the contamination.

What effects can pathogenic strains cause?

Symptoms depend on the pathotype. Diarrheal infections can cause abdominal cramps, watery or bloody diarrhea, vomiting, and sometimes moderate fever.

Infections by STEC can progress to hemolytic uremic syndrome, or HUS. This serious complication can lead to kidney failure, hemolytic anemia, and a decrease in platelets.

• persistent or bloody diarrhea;
• severe abdominal pain;
• repeated vomiting;
• signs of dehydration;
• decreased urination;
• paleness, unusual fatigue, or bruising;
• blood in the urine.

Who is most vulnerable?

• children under five years old;
• the elderly;
• immunocompromised individuals;
• people suffering from certain chronic diseases;
• travelers exposed to insufficiently safe water or food;
• people using uncontrolled private water;
• people exposed during an outbreak or through contact with carrier animals.

Young children and the elderly are at an increased risk of dehydration and severe complications during certain E. coli infections.

What is the regulatory limit for E. coli in drinking water?

European Directive 2020/2184 and French regulations set a quality limit of 0 E. coli per 100 mL for water intended for human consumption. For water in bottles or containers, the European unit is the number per 250 mL.

E. coli and intestinal enterococci are key microbiological parameters. A result higher than zero in drinking water is a non-compliance requiring a rapid risk assessment.

What is the difference between E. coli and coliform bacteria?

Coliform bacteria constitute a broader group. Some can originate from fecal matter, while others live in soils, plants, biofilms, or the environment.

A positive coliform result with a negative E. coli result remains abnormal in drinking water and may indicate network degradation or environmental contamination. It should not be interpreted as equivalent to proven fecal contamination.

How to sample water for E. coli testing?

Sampling quality is crucial. Contamination of the bottle, hands, or faucet can produce a false-positive result. Conversely, inadequate rinsing or disinfection can alter the target result.

1. use a sterile bottle provided by the laboratory;
2. choose the sampling point according to the analysis objective;
3. remove faucet accessories when the protocol requires it;
4. clean or disinfect the point according to the laboratory's instructions;
5. let the water run for the prescribed duration;
6. avoid touching the neck, cap, or inside of the bottle;
7. leave the required air gap without rinsing the bottle;
8. keep cold and transport to the laboratory quickly.

Bottles intended for chlorinated water generally contain a neutralizing agent, such as sodium thiosulfate. The bottle must not be rinsed before sampling.

How is E. coli tested in the laboratory?

French regulations recognize the NF EN ISO 9308-1 or NF EN ISO 9308-2 methods for analyzing E. coli and coliform bacteria in water intended for human consumption.

Regulatory culture allows for the detection of the E. coli indicator. Determining a pathotype such as STEC requires additional analyses focusing on toxins, their genes, or other virulence factors.

How to interpret an analysis result?

Result: absence in 100 mL

The result complies with requirements for the E. coli parameter in the analyzed sample. It does not constitute an absolute guarantee for the entire installation or against intermittent contamination.

Result: presence in 100 mL

Any detection in drinking water is non-compliant. The context must be confirmed, consumers protected, the origin investigated, and other microbiological parameters checked.

Unexpected or isolated result

A sampling error remains possible, but it should not be assumed without investigation. A new sample may supplement the assessment without delaying necessary precautionary measures.

How to interpret E. coli in raw water or a private well?

Raw water intended for treatment may contain microorganisms before potabilization. The values applicable to the resource are therefore not the same as the limit for distributed water.

For a private well used as drinking water, the presence of E. coli indicates fecal contamination and makes the water unfit for consumption without corrective measures and appropriate monitoring. Simple spot disinfection is not enough if the cause—crack, runoff, nearby sewage, or wellhead defect—is not corrected.

• inspect the wellhead, casing, and seals;
• check surface runoff;
• check the proximity of septic tanks and livestock areas;
• analyze after flooding, work, or changes in taste;
• have the return to compliance confirmed before resuming consumption.

How does collective treatment eliminate E. coli?

Control relies on several successive barriers. The exact treatment depends on the quality of the resource and the level of contamination.

1. Resource protection: limit fecal inputs.
2. Coagulation and clarification: remove particles and some microorganisms.
3. Filtration: reduce bacteria and turbidity.
4. Disinfection: inactivate remaining bacteria.
5. Protected storage: avoid recontamination after treatment.
6. Residual and monitoring: monitor the network up to the consumer.

Cloudy water can protect microorganisms and reduce the effectiveness of disinfection. Turbidity, flow rate, contact time, dose, and network integrity must be monitored.

Which disinfectants are effective against E. coli?

E. coli is generally sensitive to disinfectants used for drinking water when the dose and contact time are appropriate. Chlorine, chlorine dioxide, ozone, and ultraviolet light can contribute to its inactivation.

Disinfection does not correct permanent intrusion, a broken pipe, a poorly protected well, or open storage. The source of the contamination must be eliminated.

Which filtration systems can retain E. coli?

E. coli generally measures a few micrometers. Some microfiltration, ultrafiltration, nanofiltration, or reverse osmosis membranes can act as a physical barrier if their integrity is proven.

Does boiling inactivate E. coli?

Yes. When an authority recommends boiling water, their instructions must be followed exactly. In the absence of more precise local 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. if the water is cloudy, let it settle then pre-filter it;
2. bring the water to a full boil;
3. maintain boiling for the recommended duration;
4. let cool without adding unsafe ice;
5. store in a clean, closed, and identified container.

Boiling inactivates microorganisms but does not remove chemical pollutants and does not repair the installation causing the contamination.

How to evaluate a domestic device against E. coli?

A general claim such as "antibacterial," "purifies water," or "fine filtration" is not enough . You must look for precise microbiological performance, obtained on the complete model and under defined conditions.

• identify the exact model and tested cartridge;
• verify the microorganism used during the test;
• examine the reduction level obtained and the test duration;
• check the flow rate, treated volume, and quality of the test water;
• verify airtightness and the risk of medium bypass;
• respect the planned cleaning and replacement;
• avoid recontamination of the tank, faucet, and hands.

How to prevent E. coli contamination?

• protect catchments and wellheads against runoff;
• maintain sanitation facilities in good condition;
• analyze a well after flooding, work, or intrusion;
• secure tanks, vents, and overflows;
• avoid cross-connections and water backflow;
• disinfect and check networks after work;
• wash hands after using the toilet and before preparing food;
• do not drink untreated river, spring, or rainwater;
• maintain domestic devices according to their instructions.

What to do if E. coli is detected in water?

1. consider the water non-compliant for consumption;
2. follow instructions from the distributor, town hall, or health authority;
3. use another safe water source or boil according to instructions;
4. do not prepare ice cubes, baby bottles, or food with the restricted water;
5. quickly investigate the origin of the contamination;
6. inspect the resource, treatment, storage, and network;
7. perform the necessary corrective measures and disinfection;
8. carry out control analyses before resuming normal use.