What are coliform bacteria?

Coliforms do not correspond to a single bacterial species. It is a group historically defined by culture characteristics and, in modern methods, by certain enzymatic activities, notably β-galactosidase.

They are generally described as Gram-negative, non-spore-forming, facultatively anaerobic bacilli. Their exact definition may vary slightly depending on the analysis method, culture medium, and enzymatic criteria used.

Which genera can belong to the coliform group?

Depending on the definitions and methods used, the group may include species belonging notably to the Escherichia, Enterobacter, Klebsiella, and Citrobacter genera, and sometimes other related genera.

A regulatory "coliform bacteria" analysis generally does not provide the names of all species present. It provides a count of the group as defined by the method.

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

E. coli belongs to the coliform group, but not all coliforms are E. coli. The term "fecal coliforms" or "thermotolerant coliforms" refers to a subgroup used historically, but E. coli is now preferred to characterize fecal contamination of drinking water.

Why test for coliforms in drinking water?

Total coliforms are mainly used as operational indicators. Their absence at the treatment output helps verify that barriers are functioning properly. Their appearance in the network can signal an intrusion, bacterial regrowth, a loss of disinfectant, or a storage defect.

• control of general treatment effectiveness;
• verification of disinfection;
• monitoring the integrity of the distribution network;
• detection of contamination after work or a break;
• assessment of the vulnerability of a well or catchment;
• identification of bacterial regrowth in a biofilm or reservoir.

Health Canada emphasizes that total coliforms are widespread in both fecal and non-fecal environments. They are therefore less suited than E. coli for specifically identifying fecal pollution, but remain useful for monitoring the operation of a drinking water system.

How do coliform bacteria get into the water?

Their origin can be natural, environmental, or linked to human activities.

• soil, plants, leaves, roots, and organic matter;
• surface water and sediments;
• human or animal fecal matter;
• wastewater or faulty sanitation;
• agricultural or urban runoff;
• intrusion into a well, reservoir, or pipe;
• biofilms present in the network;
• contamination from the tap or sampling equipment.

Raw surface water may naturally contain coliforms. In treated water, their detection is more concerning because it may reveal an insufficiency or degradation after treatment.

Can coliform bacteria multiply in a network?

Yes, certain environmental species can persist or multiply in biofilms, reservoirs, or pipes when conditions are favorable.

• high temperature;
• long residence time and stagnation;
• low disinfectant residual;
• biodegradable organic matter;
• corrosion, scale, and deposits;
• dead ends or oversized pipes;
• poorly maintained reservoirs.

Health Canada and the EPA describe total coliforms as indicators of water quality degradation in the network, particularly through bacterial regrowth or post-treatment contamination.

Which situations favor their detection?

Are coliform bacteria dangerous to health?

Most total coliforms tested as indicators are not considered directly responsible for illness in healthy people. Their presence mainly indicates that the water may have been insufficiently treated, contaminated, or degraded in the network.

However, certain species belonging to coliform genera can be opportunistic agents in specific medical contexts. Routine analysis does not allow for automatic determination of the species, virulence, or antibiotic resistance.

What is the regulatory value for coliform bacteria?

In France, coliform bacteria have a quality reference of 0 in 100 mL. This qualification is different from the "quality limit" applicable to E. coli and intestinal enterococci.

Directive (EU) 2020/2184 also classifies coliform bacteria among indicator parameters and sets a value of 0 numbers in 100 mL. For water in bottles or containers, the European unit is the number in 250 mL.

How to collect a water sample to test for coliforms?

The sampling procedure must make it possible to distinguish actual water contamination from contamination of the tap or equipment.

1. use a sterile bottle provided by the laboratory;
2. choose the sampling point according to the analysis objective;
3. remove the aerator or accessories when the protocol requires it;
4. clean or disinfect the tap according to the instructions;
5. let the water run for the prescribed duration;
6. do not touch the neck, the cap, or the inside of the bottle;
7. do not rinse the bottle if it contains 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 and prevent it from continuing to act during transport.

How are coliform bacteria analyzed?

The French decree of October 19, 2017, recognizes the NF EN ISO 9308-1 and NF EN ISO 9308-2 methods for the joint analysis of E. coli and coliform bacteria.

The 9308-1 method classically identifies coliforms by β-galactosidase activity and E. coli by the combination of enzymatic criteria. The 9308-2 method provides a statistical estimate of the most probable number.

How to interpret a coliform result?

Coliforms absent and E. coli absent

The sample is compliant for these two parameters. The result remains limited to the point, the time, and the volume analyzed.

Coliforms present and E. coli absent

This situation may indicate environmental contamination, regrowth in the network, treatment failure, poorly protected storage, or contamination of the sampling point. Investigation and further sampling are necessary.

Coliforms present and E. coli present

The presence of E. coli indicates fecal contamination and constitutes a non-compliant quality limit. Rapid health measures are required.

Repeated detections at the same point

These point to a local problem: biofilm, flexible hose, terminal filter, rarely used branch, or a defect in the water point.

Detections at multiple points

These may reveal a problem with treatment, the reservoir, disinfection, or the general integrity of the network.

What does the presence of coliforms in raw water or a well mean?

Since total coliforms are naturally present in soils, plants, and surface water, their detection in a raw resource does not necessarily indicate fecal contamination.

In protected groundwater or at the outlet of a well used without disinfection, their presence may signal vulnerability to infiltration from the surface, a construction defect, or contamination after maintenance work.

The CDC recommends that private well owners test for total coliforms at least annually, in addition to other parameters adapted to the local context.

• inspect the wellhead and casing;
• check the slope of the land and drainage;
• check the tightness of connections;
• check for proximity to a sewage system or livestock;
• analyze after a flood or construction work;
• correct the cause before a one-off disinfection.

How does collective treatment manage coliforms?

Coliforms are generally sensitive to processes correctly designed to produce microbiologically safe water.

1. Resource protection: limit intrusions and organic matter input.
2. Clarification: reduce particles and turbidity.
3. Filtration: remove a significant portion of bacteria.
4. Disinfection: inactivate remaining bacteria.
5. Protected storage: avoid contamination after treatment.
6. Network monitoring: pressure, residual, temperature, and analyses.

Detection of coliforms at the exit of a plant may indicate insufficient treatment. Their appearance only in the network may point more toward contamination after treatment or regrowth.

Which disinfectants are effective against coliforms?

Chlorine, chloramines, chlorine dioxide, ozone, and ultraviolet light can reduce or inactivate coliform bacteria when application conditions are controlled.

One-off disinfection can reduce contamination without removing its cause. A leak, an open reservoir, a dead end, or a colonized filter must be corrected.

Which filtration systems can retain coliforms?

Coliforms are micrometer-sized bacteria. Several membrane processes can constitute a physical barrier if the integrity of the system is ensured.

Does boiling inactivate coliform bacteria?

Yes. When an authority issues a boil water advisory, its instructions must be followed. 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.

Boiling inactivates microorganisms but does not remove chemical contaminants and does not correct the failure of the well, storage, or network.

How to evaluate a domestic coliform removal device?

A "bacterial," "fine filtration," or "purified water" claim does not constitute proof of microbiological reduction.

• identify the complete model that was tested;
• verify the bacteria used during the protocol;
• examine the reduction level and duration of the test;
• verify the flow rate and volume treated;
• check the absolute pore size and sealing;
• comply with replacement and disinfection of the device;
• protect the reservoir and tap against recontamination.

How to prevent their appearance in drinking water?

• protect catchments and wells from runoff;
• maintain sufficient pressure in the network;
• secure reservoirs, vents, and overflows;
• avoid cross-connections and backflow;
• disinfect and monitor the network after work;
• limit stagnation and remove dead ends;
• clean reservoirs and terminal points;
• maintain domestic filters according to the manual;
• analyze a well after flooding, repairs, or unusual changes.

What to do if coliform bacteria are detected?

1. verify the E. coli result immediately;
2. inform the network manager or the competent authority;
3. evaluate the need for restrictions according to the health context;
4. carry out confirmation samples without delaying precautions;
5. inspect the treatment, the reservoir, and the network;
6. check for recent work, pressure losses, or stagnation;
7. correct defects and disinfect if necessary;
8. confirm return to normal with new analyses.