Bacteria in water: contamination, risks, analysis and treatment
Bacteria are microorganisms naturally present in soil, water, organic matter, animals and humans. Most are not dangerous, but some can cause infections or reveal fecal contamination, treatment failure, network intrusion or growth within installations. The microbiological quality of drinking water is therefore monitored using indicator bacteria, notably Escherichia coli and intestinal enterococci.
- Nature
- Living microorganisms
- Primary indicator
- Escherichia coli
- France and EU limit
- 0 E. coli / 100 mL
- Analysis
- Sterile sampling and culture
What are the bacteria present in water?
Bacteria are microscopic organisms made up of a single cell. They are found in almost all natural environments. Water can therefore contain bacteria without being automatically dangerous.
The health concern depends on the species or group detected, its concentration, its ability to cause disease, the route of exposure and the health status of the exposed individuals.
In drinking water control, laboratories do not systematically test for all possible pathogens. They use indicator organisms whose presence signals a contamination or malfunction requiring investigation.
Keep in mind: the term "bacteria" encompasses very different organisms. A result must always specify the parameter analyzed: E. coli, enterococci, coliforms, revivable germs, Legionella or other bacteria being searched for.
What is the difference between an indicator bacterium and a pathogenic bacterium?
| Category | Role in analysis | General interpretation |
|---|---|---|
| Fecal contamination indicator bacterium | E. coli and intestinal enterococci. | Their presence indicates that recent or persistent fecal contamination is possible and that pathogens may also be present. |
| Network performance indicator bacterium | Coliform bacteria and total viable count. | An anomaly may indicate a treatment failure, an intrusion, stagnation, a biofilm or an unusual modification in the network. |
| Pathogenic bacterium | Bacterium capable of causing disease under certain conditions. | The risk depends on the strain, the dose, the route of exposure and the vulnerability of the person. |
| Opportunistic bacterium | Organism that can grow in wet networks or equipment. | The risk mainly concerns certain fragile people and certain routes of exposure, particularly the inhalation of aerosols for Legionella. |
The absence of E. coli in a sample does not prove the absolute absence of all bacteria, viruses or parasites. It indicates that this marker was not detected in the analyzed volume and under the sampling conditions.
Which bacteria can be tested for in water?
| Bacterium or group | Role or frequent origin | Health importance |
|---|---|---|
| Escherichia coli – E. coli | Intestinal bacterium used as an indicator of fecal contamination. | Most strains are harmless, but some are pathogenic. Its detection in drinking water constitutes non-compliance. |
| Intestinal enterococci | Human or animal intestinal bacteria, relatively resistant in the environment. | Regulatory indicators of fecal contamination. Their detection in drinking water constitutes non-compliance. |
| Coliform bacteria | Group including bacteria of fecal and environmental origin. | Used mainly to monitor treatment efficiency and network integrity. Not all are fecal or pathogenic. |
| Campylobacter | Animal fecal matter, runoff, inadequately treated water. | Can cause gastroenteritis and other complications. |
| Salmonella | Human or animal fecal contamination. | Some species or strains cause intestinal infections or enteric fevers. |
| Shigella | Contamination of human origin and fecal-oral transmission. | Can cause shigellosis, with sometimes severe diarrhea. |
| Toxigenic Vibrio cholerae | Contaminated water or food in areas where cholera circulates. | Responsible for cholera when a toxigenic strain is involved. |
| Legionella | Hot water networks, installations with stagnation and aerosols. | Risk mainly through inhalation of fine contaminated droplets, not by usual ingestion of water. |
| Pseudomonas aeruginosa | Wet environments, biofilms, equipment and networks. | Opportunistic bacterium particularly monitored in certain facilities, uses and packaged waters. |
How do bacteria get into the water?
Fecal contamination
Human or animal waste can contaminate a resource through wastewater, sewage leaks, manure spreading, runoff, flooding or infiltration into a poorly protected catchment.
Treatment failure
Poor clarification, insufficient filtration, inadequate disinfection or high turbidity can reduce the effectiveness of treatment.
Network intrusion
Pipe breakage, pressure drops, unprotected connections, poorly maintained reservoirs or construction work can allow contaminated water to enter the network.
Stagnation and biofilms
Some bacteria can persist or multiply in deposits, biofilms, dead ends, little-used equipment and installations where temperature and nutrients are favorable.
Private wells and springs
A shallow, cracked or poorly covered well located near a septic tank, a farm or a flood zone presents an increased risk of microbiological contamination.
Why are biofilms important in water networks?
A biofilm is a community of microorganisms attached to a surface and surrounded by a protective matrix. It can grow on the walls of pipes, seals, reservoirs, taps and equipment.
Biofilms do not automatically mean that the water contains pathogenic bacteria. However, they can promote the persistence of certain species, protect cells against unfavorable conditions and lead to variations in microbiological quality.
Controlling biofilms relies in particular on the quality of treatment, limiting stagnation, maintenance of structures, temperature management and, in disinfected networks, the maintenance of an appropriate residual.
What are the health risks linked to bacteria in water?
The effects depend on the bacterium, the strain, the amount ingested or inhaled, the duration of exposure and the person's sensitivity.
Possible symptoms include:
- diarrhea, nausea, vomiting and abdominal pain;
- fever and dehydration;
- intestinal or systemic infections;
- renal or neurological complications for certain infections;
- respiratory infections in the case of Legionella inhaled in an aerosol;
- opportunistic infections in certain fragile people.
Microbial contamination of fecal origin represents a short-term risk. Clear water, without odor and normal taste, can nevertheless be contaminated.
Appearance, smell and taste do not allow for the confirmation of the microbiological safety of water. Only an appropriate analysis can search for indicators or targeted agents.
Which people may be more vulnerable?
- infants and young children;
- the elderly;
- immunocompromised people;
- people with severe chronic illnesses;
- hospitalized patients or those residing in certain care facilities;
- people exposed to contaminated aerosols in the case of Legionella.
In the event of a restriction notice or non-compliance, the health authority's recommendations may differ depending on the uses and the populations concerned.
What are the microbiological limits for drinking water?
French regulations and the European directive set a value of 0 per 100 mL for E. coli and intestinal enterococci in water intended for human consumption.
| Parameter | Value in France | Nature of the value |
|---|---|---|
| Escherichia coli | 0 / 100 mL | Microbiological quality limit. |
| Intestinal enterococci | 0 / 100 mL | Microbiological quality limit. |
| Coliform bacteria | 0 / 100 mL | Quality reference and operational indicator. |
| Spores of sulfite-reducing anaerobic microorganisms | 0 / 100 mL | Quality reference measured when water is surface water or influenced by surface water. |
| Culturable aerobic germs at 22 °C and 36 °C | No variation greater than a factor of 10 from the usual value | Indicator of unusual change in microbiological quality. |
The values applicable to drinking water should not be confused with those for bathing water, swimming pools, raw water, or bottled water. The type of water and the sampling point must always be specified.
How should a microbiological result be interpreted?
| Result | General interpretation | Action to consider |
|---|---|---|
| E. coli detected | Indicator of fecal contamination and drinking water non-compliance. | Alert the water manager, confirm the result, and apply the measures defined by the health authority. |
| Enterococci detected | Indicator of fecal contamination and drinking water non-compliance. | Identify the source, check the treatment and the network, then perform control analyses. |
| Coliforms without E. coli | May indicate an intrusion, a treatment defect, a biofilm, or contamination of the sample. | Examine the sampling point, repeat the analysis, and inspect the network or installation. |
| Increase in culturable germs | Unusual change that may be related to stagnation, temperature, maintenance, or an evolution in the network. | Compare with usual values and look for recent changes. |
| Single negative result | No target organism detected in the analyzed volume at that moment. | Does not rule out intermittent contamination or contamination located at another point in the network. |
How to carry out a reliable microbiological sample?
The sampling must comply with the laboratory's instructions. A handling error can introduce bacteria into the bottle or, conversely, prevent their detection.
- use a sterile bottle provided or validated by the laboratory;
- do not touch the inside of the cap or the neck of the bottle;
- precisely identify the point, date, and time of sampling;
- follow the instructions for purging or disinfecting the tap;
- protect the sample from heat and light;
- respect the required temperature and transport time;
- report any disinfectant treatment in order to use the appropriate neutralizer;
- have the regulatory control performed by a competent laboratory.
A sample taken from a tap without removing a dirty aerator does not serve the same purpose as a sample taken after disinfecting the point. The protocol must be chosen according to whether the network, the tap, or the internal installation is being studied.
What methods are used to analyze bacteria?
Regulatory methods are mainly based on the culture and enumeration of bacteria capable of growing under defined conditions.
| Parameter | Reference method cited in France | General principle |
|---|---|---|
| E. coli and coliform bacteria | NF EN ISO 9308-1 or NF EN ISO 9308-2 | Membrane filtration or most probable number method. |
| Intestinal enterococci | NF EN ISO 7899-2 | Membrane filtration and selective culture. |
| Culturable germs at 22 °C and 36 °C | NF EN ISO 6222 | Culture and colony counting under defined conditions. |
| Pseudomonas aeruginosa | NF EN ISO 16266 | Filtration, selective culture, and confirmation. |
| Legionella spp. and Legionella pneumophila | NF T 90-431 | Concentration, culture, and identification according to the protocol. |
Molecular methods
PCR and quantitative PCR can search for genetic material with high sensitivity. However, they do not always provide the same information as a culture, because DNA can be detected even when the viability of the bacterium is not proven.
Limits of the analysis
The result depends on the volume, the sampling point, the transport time, the method, the detection limit, and the potentially intermittent nature of the contamination.
How do collective installations control bacteria?
Microbiological safety is based on a multi-barrier approach, from the protection of the resource to the tap.
- protection of the catchment area and management of discharges;
- coagulation and flocculation when water quality requires it;
- sedimentation or flotation;
- media or membrane filtration;
- disinfection adapted to water quality;
- maintenance of network integrity and pressure;
- management of reservoirs, deposits, works, and incidents;
- analytical monitoring and safety management plans.
A single step is not necessarily sufficient. Turbidity, organic matter, pH, temperature, contact time, and the condition of the structures influence performance.
Chlorine, ozone, and UV: how does disinfection work?
Chlorination
Chlorine and certain chlorine derivatives inactivate many bacteria. Their effectiveness depends notably on the dose, contact time, pH, temperature, and the disinfectant demand of the water. A residual can help protect the network against certain recontaminations.
Ozone
Ozone is a powerful oxidant used in some installations. It does not provide a lasting residual in the network and must be integrated into a properly designed treatment chain.
Ultraviolet
UV can inactivate bacteria without adding chemicals. Performance depends on the dose received, water transmittance, turbidity, lamp cleanliness, and flow rate. UV does not leave residual protection after treatment.
Cloudy water or water containing particles can protect microorganisms and reduce the effectiveness of certain disinfection methods. Pre-treatment may be necessary.
Can filtration retain bacteria?
Filtration can reduce bacterial load when the barrier is appropriate, intact, and properly installed. Performance does not depend solely on an advertised pore diameter.
Media filtration
Granular filters can help retain particles and microorganisms, particularly in a chain including coagulation, clarification, and disinfection.
Microfiltration and ultrafiltration
Membranes can form an effective barrier against many bacteria when their integrity is monitored. Defects, seals, bypasses, and ruptures can negate the expected performance.
Microporous ceramic
A ceramic can retain bacteria if its structure, effective threshold, and assembly have been validated by a microbiological test. A crack, a defective seal, incorrect assembly, or bypassing the cartridge can let untreated water through.
Activated carbon
Activated carbon is mainly used to adsorb certain chemical contaminants and improve taste or odor. It should not be presented as bacterial disinfection without a specific test of the complete system. Poorly maintained media can also harbor a biofilm.
Does boiling eliminate bacteria?
Bringing water to a rolling boil inactivates pathogenic bacteria, as well as many viruses and protozoa. Boiling is a useful emergency measure when authorities recommend boiling water.
After treatment, the water must cool in a clean, closed container protected against recontamination.
Boiling does not remove non-volatile chemical contaminants and can concentrate certain substances as water evaporates. A chemical alert should therefore not be treated as a simple microbiological alert.
Comparison of technologies against bacteria
| Technology | General potential | Main limitations |
|---|---|---|
| Boiling | High inactivation when performed correctly | No removal of chemical contaminants and risk of recontamination. |
| Chlorination | Inactivation of many bacteria | Depends on dose, contact time, pH, turbidity, and water demand. |
| Ultraviolet | High inactivation with validated dose | No residual; reduced effectiveness if water is cloudy or if the lamp is poorly maintained. |
| Microfiltration | Variable to high depending on the membrane | Integrity, effective pore size, seals, and risk of bypass. |
| Ultrafiltration | High for many bacteria | Fouling, membrane defects, and maintenance. |
| Reverse osmosis | High potential with intact membrane | Pressure, sealing, maintenance, and post-membrane contamination. |
| Ceramic | Variable depending on system and test | Cracks, seals, effective threshold, and potential lack of testing. |
| Activated carbon only | Not considered reliable disinfection without validation | Biofilm possible and no demonstrated microbiological barrier. |
| Water softener | Not designed to eliminate bacteria | Maintenance essential; does not replace disinfection. |
How to evaluate a device against bacteria?
A claim of microbiological reduction must be based on a test of the complete system and not solely on the composition of the media.
| Element to check | Why is it important? |
|---|---|
| Tested organism | Performance on one bacterium does not prove the same performance on all. |
| Strain or surrogate organism | The protocol must precisely identify the microorganism used. |
| Inlet and outlet concentration | Allows calculation of actual reduction. |
| Log reduction | A 1, 2, 3, or 6 log reduction does not correspond to the same level of performance. |
| Total volume treated | Performance must be verified beyond the first few liters. |
| Flow and pressure | They influence passage, contact time, and potential defects. |
| End of device life | Initial performance does not demonstrate performance until replacement. |
| Integrity and risk of bypass | Water must not bypass the filtering barrier. |
| Laboratory and protocol | They must be identifiable, documented, and adapted to the stated use. |
| Storage after treatment | Correctly treated water can be recontaminated in a dirty container. |
In the absence of a specific microbiological test, no reduction in bacteria should be inferred solely from the material, nominal fineness, or a test covering chemical contaminants.
How to monitor a private well or spring?
Private groundwater can be contaminated without any visible change. The risk increases after flooding, construction work, periods of heavy rain, sewage system failure, or changes to the catchment area.
- have the wellhead, casing, cover, and drainage inspected;
- keep pollution sources away and check the sewage system;
- carry out a microbiological analysis in a competent laboratory;
- repeat the analysis after an incident or changes in the water;
- do not conclude that water is potable based solely on taste or clarity;
- identify and correct the source before relying solely on treatment.
The frequency of analysis should be adapted to the local context, usage, and recommendations from health authorities.
What to do in the event of an alert or positive result?
- stop using the water for drinking if recommended by the authority or laboratory;
- use alternative water or follow the communicated boil water advisory;
- do not improvise disinfectant dosages without a validated protocol;
- notify the network manager, owner, or competent authority;
- investigate the source: catchment, treatment, storage, network, or tap;
- implement corrective measures, then conduct follow-up analyses;
- comply with specific recommendations for vulnerable individuals.
A positive result should not be "fixed" simply by replacing a cartridge. You must identify the contamination pathway and verify the safety of the entire system.
Frequently asked questions about bacteria in water
Can clear water contain bacteria?
Yes. Most microbiological contaminations do not necessarily alter the color, odor, or taste of the water.
What does E. coli in a water analysis mean?
E. coli is an indicator of fecal contamination. Its presence in drinking water constitutes non-compliance and requires investigation.
Are all coliform bacteria dangerous?
No. The group includes environmental and fecal bacteria. However, their detection can signal a treatment or network issue.
What is the regulatory limit for E. coli?
The limit is 0 E. coli per 100 mL of water intended for human consumption in France and the European Union.
Does a negative test guarantee the absence of all bacteria?
No. It only indicates that the organism being tested for was not detected in the analyzed volume and under the sampling conditions.
Does boiling water kill bacteria?
A rolling boil can inactivate pathogenic bacteria. The specific instructions of the health authority must be followed.
Does activated carbon remove bacteria?
It should not be considered reliable disinfection without a specific microbiological test of the complete system.
Can a ceramic filter retain bacteria?
It can act as a barrier when its structure and assembly have been validated by a microbiological test. Cracks and assembly leaks compromise performance.
Are UV systems effective against bacteria?
Yes, when the dose, flow rate, water quality, and lamp maintenance comply with the system's specifications.
Should well water be analyzed?
Yes. A microbiological analysis is necessary to assess its quality, especially after flooding, construction, or changes to the catchment area.
Is Legionella primarily transmitted by drinking water?
The main risk is the inhalation of contaminated aerosols, which can come from certain hot water installations.
Can chemically contaminated water be disinfected by boiling it?
No. Boiling targets microorganisms but does not necessarily make water containing dangerous chemicals safe.
Related parameters and contaminants
Interpreting microbiological contamination may require studying other parameters that indicate intrusion, runoff, or changes in the resource.
Scientific and administrative sources
This fact sheet is based on regulatory texts, public bodies, and recognized health institutions. No commercial blogs, comparison sites, or affiliate sites are used as primary microbiological sources.
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Ministry of Health – Drinking Water Quality
Official information on microbiological quality, causes of contamination, and the use of E. coli and enterococci as indicators.
Visit the ministry's official page -
France – Drinking water quality limits and references
Decree of December 30, 2022, establishing, among other things, the 0/100 mL limits for E. coli and intestinal enterococci.
Consult the text on Légifrance -
France – Methods for sanitary water control analysis
Official references for E. coli, coliforms, enterococci, revivable germs, Pseudomonas, and Legionella.
Consult the methods on Légifrance -
European Union – Directive (EU) 2020/2184
European text regarding the quality of water intended for human consumption and microbiological parameters.
Consult the directive on EUR-Lex -
World Health Organization – Drinking water
Summary on the risks associated with microbial water contamination and waterborne diseases.
Consult the WHO fact sheet -
World Health Organization – Water- and sanitation-related pathogens
Scientific reference documents on bacteria, viruses, and protozoa of public health importance.
Consult the WHO documents -
World Health Organization – Escherichia coli
Health information on E. coli, pathogenic strains, and transmission routes.
Consult the WHO fact sheet -
ANSES – Legionella and legionellosis
Information on Legionella, water networks, and exposure via aerosol-producing installations.
Consult the ANSES page -
World Health Organization – Boiling water
Technical note on the inactivation of bacteria, viruses, and protozoa through a rolling boil.
Consult the WHO technical note -
WHO – Evaluating household water treatment technologies
International program to evaluate the microbiological performance of household water treatment technologies.
Consult the WHO program -
US EPA – Revised Total Coliform Rule
Regulatory information on the use of coliforms and E. coli as indicators of drinking water network integrity.
Consult the official EPA page
