What do coliforms and E. coli in water mean?
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Key points
- Coliforms and E. coli are indicator organisms: a positive result signals a route for contamination into the water, most often at or after the cold water storage tank.
- The drinking-water standard is absence in 100 millilitres. Any detection at a tap is a failure to investigate, not a number to watch.
- A clear coliform result tells you nothing about legionella. The organisms come from different sources, are measured differently, and are controlled in different ways.
- The correct response to a positive is to find the route in, clean and disinfect, and resample to confirm, recording each step.
- Tank integrity checks for coliform control and legionella risk assessment look at the same physical defects, so the two inspections naturally run together.
Coliforms are a broad group of bacteria used as indicators of contamination in drinking water, and E. coli is the member of that group that points specifically to recent faecal contamination. The standard for water supplied for drinking and domestic use is that neither should be detectable in a 100-millilitre sample, so any positive result at a building tap is a failure that needs investigating, not a figure to monitor. They matter to a duty holder for a different reason than legionella does, and mixing the two up produces the wrong response in both directions.
What coliforms and E. coli actually indicate
The point of testing for coliforms is not that they are usually dangerous in themselves. It is that they are easy to culture and their presence proves a pathway exists. If bacteria from soil, insects or worse can get into the water, so can other things. E. coli tightens that signal: it lives in the gut, so finding it means the contamination is recent and faecal in origin, which raises the stakes considerably. A broader coliform count without E. coli more often means environmental ingress, dirt washed in during works, or regrowth in a part of the system that was not properly disinfected.
As a duty holder you will most often meet these organisms in three places: routine sampling of a cold water storage tank, sampling required on a private water supply, and verification samples taken after a system has been cleaned and disinfected. In each case the question the result answers is the same: is the system keeping contamination out?
The critical difference from legionella
This is the section that matters most, because the two are routinely confused. Legionella is an environmental organism that arrives in trace amounts in the incoming mains and grows inside the building: in warm, stagnant water, in biofilm, and around scale and sediment. It needs no contamination event and no faecal source. Coliforms work the other way round: their presence means something got in from outside through a defect.
Everything downstream of that difference is different too. Coliforms are judged present-or-absent in 100 millilitres; legionella is counted as colony-forming units per litre against published action levels. Coliform control is about tank integrity, screened vents, sound lids, backflow protection and disinfection; legionella control is about temperature, turnover and cleaning. So a tank with a missing vent screen can fail coliforms while its temperatures are textbook, and a perfectly sealed system can still grow legionella in a warm dead leg. Each test only answers its own question.
A nil coliform result is not reassurance about legionella, and a clear legionella sample is not reassurance about coliforms. Each test only answers its own question.
What to do when a sample is positive
Treat any detection as an actionable failure. The sequence is straightforward:
- Confirm it. Resample to rule out a sampling error, using a clean technique and a sterile bottle from the laboratory.
- Find the route in. Inspect the tank lid, seals, vent and overflow screens for gaps, damage, insects or debris. Check whether any recent plumbing work could have introduced dirt, and look for cross-connections that could let non-potable water back into the supply.
- Fix the defect, then clean and disinfect. Repair the physical fault first, then clean and disinfect the tank and distribution following the standard chlorination procedure. Disinfecting without fixing the route in only buys time.
- Resample to confirm. A clear confirmation sample is the evidence the problem is resolved.
While the investigation is open, take sensible precautions with water used for drinking and follow the advice of the laboratory or water company on whether use should be restricted.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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Where coliform sampling fits alongside legionella control
The two regimes overlap in the physical inspection. The defects that let coliforms in, such as a damaged tank lid or an unscreened overflow, are exactly the defects a legionella risk assessment asks you to record, because anything that admits insects and debris also admits organic matter that feeds biofilm. Our free risk assessment builder walks through tank condition and contamination routes as part of the same exercise. Keep every result, the cause found, the remedial work and the confirmation sample in your log book, alongside the temperature and flushing records. The general principles of taking and interpreting samples are covered in our guide to legionella testing, and records of this kind should be kept for at least five years so the history of the system can be shown.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What does it mean if coliforms are found in tap water?
Coliforms are indicator organisms, so a positive result means contamination has found a route into the water rather than that the coliforms themselves are the whole hazard. The usual sources in a building are a compromised cold water storage tank (a missing or damaged lid, an unscreened vent or overflow), dirt entering during plumbing work, or a cross-connection letting non-potable water back into the system. The response is to find and fix the route in, clean and disinfect the affected part of the system, and resample to confirm it is clear.
What is the limit for E. coli in drinking water?
The drinking-water standard is that E. coli, and coliform bacteria generally, should not be detectable in a 100-millilitre sample of water supplied for drinking or domestic use. There is no acceptable count: any detection is a failure and triggers investigation. This is a different style of standard from legionella, which is measured as colony-forming units per litre with action levels rather than a simple present-or-absent test.
Does E. coli in water mean legionella is present?
No. E. coli and legionella come from different sources and behave in different ways. E. coli signals recent contamination from outside the system, usually through a defect that has let dirt or sewage-derived material in. Legionella is an environmental organism that grows inside the system itself, in warm stagnant water and biofilm, and needs no faecal source. One result tells you nothing about the other, which is why the two are sampled, interpreted and controlled separately.
Can you have legionella with a clear coliform result?
Yes, and this is a common misunderstanding. A well-sealed tank and sound pipework can pass every coliform test while warm dead legs, blended water and scale downstream still grow legionella. Coliform tests answer a question about contamination getting in; legionella control is about conditions inside the system. A duty holder needs both: sound tank integrity to keep contamination out, and temperature and stagnation control to stop legionella growing within.
How do you clear coliforms from a cold water tank?
Start by finding how the contamination got in, because disinfecting without fixing the defect only buys time. Inspect the lid, seals, vent and overflow screens, and look for signs of insects, debris or recent work nearby. Then empty, clean and disinfect the tank and the distribution it feeds, following the standard chlorination procedure, and resample afterwards to confirm the result is clear. Record the result, the cause found, the work done and the confirmation sample in the log book.
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