What is mycobacterium in a water system?
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Key points
- Non-tuberculous mycobacteria (NTM) are environmental bacteria that colonise building water systems and their biofilm; the Mycobacterium avium complex is the group most often discussed.
- They are opportunists: little risk to healthy people, a genuine concern for those with damaged lungs or weakened immunity, so healthcare settings give them the most attention.
- Their waxy cell wall makes them more resistant to chlorine and heat than legionella, and biofilm shields them further.
- A routine legionella sample does not detect them; they need different, slower laboratory methods.
- Control is the same discipline as legionella prevention: movement, correct temperatures, and physical removal of biofilm and scale.
Mycobacteria in a water system are almost always the non-tuberculous kind, usually shortened to NTM. They are environmental organisms that live naturally in soil and water, arrive in trace amounts in the incoming supply, and colonise the same places legionella does: the biofilm inside pipes, tanks, and shower fittings. They matter in water hygiene because they are unusually hard to kill and because the people most at risk from them are concentrated in healthcare. This page covers the organism in the system; anything clinical belongs with the NHS or a clinician.
What NTM are and where they live
The mycobacteria relevant to building water are a wide family, of which the Mycobacterium avium complex is the most commonly discussed. Like legionella, they are not really free-swimming water organisms: they live in biofilm, the slime layer that forms on any surface that stays wet. From there they can be carried in the fine aerosol a shower produces, which is the main route by which people encounter them in a building.
What sets them apart is their cell wall. Mycobacteria wrap themselves in a waxy, fatty layer that ordinary bacteria lack, and that layer is the reason for almost everything distinctive about them: they grow slowly, they shrug off chlorine concentrations that would clear most waterborne bacteria, and they tolerate heat better than legionella does. Inside biofilm, which already shields organisms from heat and disinfectant, they are harder again to shift.
Who they matter for
NTM are opportunists. For a healthy person, contact with them in water is an everyday, unremarkable event. The risk concentrates on people with existing lung damage or weakened immune systems, which is why the organisms get the most attention in healthcare and why they are managed there alongside legionella and the outlet organisms such as pseudomonas and stenotrophomonas. In those settings the framework is HTM 04-01: a water safety plan, a Water Safety Group, and decisions about sampling and remediation taken as a team rather than by one person reacting to a result.
Outside healthcare, NTM rarely drive a specific monitoring regime. The sensible reading for most duty holders is that the controls you already run for legionella are the controls that keep NTM suppressed too, because both organisms depend on the same conditions: warmth, stagnation, and biofilm.
Why routine legionella testing does not cover them
A standard legionella test cultures the sample on media designed for legionella and reads it on a timescale suited to that organism. Mycobacteria need different media and much longer incubation, and they will not show up on a legionella plate. So a clear legionella result is not evidence about mycobacteria, and if NTM are a specific concern, for example in an augmented-care unit, testing for them has to be commissioned deliberately through a laboratory that offers it. That is normally a decision for the Water Safety Group rather than a general duty holder.
What control looks like in practice
There is no separate magic regime for mycobacteria. The controls are the fundamentals, run consistently:
- Keep temperatures right. Hot water stored at 60°C and reaching outlets at 50°C, cold kept below 20°C. The figures are set with legionella in mind, and a temperature check confirms they are being met, but a system out of the growth range is hostile to NTM too. Our page on temperature control sets out the regime.
- Keep water moving. Stagnation feeds biofilm, and biofilm is the habitat. Flushing little-used outlets and removing dead legs denies both organisms their foothold.
- Remove the habitat physically. Scale and sediment are where biofilm anchors, so cleaning tanks, descaling outlets and cleaning shower heads do more against a chlorine-tolerant organism than another chemical dose.
- Treat disinfection as a support, not a cure. Because NTM tolerate chlorine well, chemical treatment alone should never be the plan. It works after the physical cleaning has stripped the protection away.
Mycobacteria share legionella's habitat but not its weaknesses. A system kept moving, kept at the right temperatures and kept physically clean suppresses both; a chlorine dose on its own suppresses neither for long.
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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Recording and the wider scheme
Whatever the organism, the evidence test is the same: the control has to be carried out and written down. Temperature checks, flushing, cleaning and any sampling belong in the log book and the schedule in the written scheme of control. NTM are a useful reminder of why that discipline exists: the organisms in a water system are a community, not a single named bacterium, and the only realistic strategy is to control the conditions all of them depend on rather than chase each one with a different chemical.
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 are non-tuberculous mycobacteria in water?
Non-tuberculous mycobacteria, often shortened to NTM, are a family of environmental bacteria that live naturally in soil and water and colonise building water systems, especially the biofilm inside pipes, tanks and shower fittings. The group most often discussed in water hygiene is the Mycobacterium avium complex. They are opportunists: they rarely affect healthy people but can cause infection in people with damaged lungs or weakened immune systems, which is why they receive most attention in healthcare.
Does chlorine kill mycobacteria in a water system?
Mycobacteria are noticeably more resistant to chlorine than most waterborne bacteria, because their waxy cell wall slows the chemical down, and they sit inside biofilm which shields them further. A chlorine level that clears coliforms from open water cannot be assumed to clear mycobacteria from an established biofilm. Control therefore leans on the same fundamentals as legionella control: deny them warmth and stagnation, remove biofilm and scale physically, and treat chemical disinfection as a supporting measure rather than a cure.
Can you test water for mycobacteria?
Yes, but it is a specialist test. Mycobacteria grow slowly and need different culture methods from a routine legionella sample, so a standard legionella laboratory result says nothing about them. Testing is usually commissioned in healthcare or outbreak situations rather than as routine monitoring, and in those settings it is coordinated through the Water Safety Group under the water safety plan.
Is mycobacterium the same as legionella?
No, they are different organisms that happen to share the same habitat. Both live in biofilm, both spread through aerosols from showers and taps, and both are controlled by keeping water moving, keeping temperatures right and keeping systems clean. The practical differences are that mycobacteria tolerate heat and chlorine better, grow more slowly, and need different laboratory methods to detect, so a clear legionella result is not evidence about mycobacteria.
Where do mycobacteria come from in a building water system?
They arrive in trace amounts in the incoming water supply, because they are normal inhabitants of the environment, and then establish wherever conditions suit them: biofilm on pipe walls, sediment in tanks, scale on heating elements, and shower heads and hoses. They do not need a contamination event. Once established they persist because their waxy wall and the biofilm around them protect them from routine disinfection.
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