What is pseudomonas in a water system?
On this page
Key points
- Pseudomonas aeruginosa is an outlet organism. It lives in the biofilm at the wet end of a system: the tap, its flow straightener, the spout and the drain beneath it, rather than throughout the pipework.
- It is opportunistic. It rarely troubles healthy people but is a real risk to seriously unwell patients, which is why it matters most in healthcare.
- Augmented care is the main concern. HTM 04-01 Part C sets out the guidance for units treating vulnerable patients such as neonatal, intensive care, burns and transplant.
- Control targets the last few metres. Because it colonises the fitting itself, control focuses on cleaning and disinfecting outlets, reducing stagnation, managing flow straighteners and TMVs, avoiding drain splashing, and fitting point-of-use filters where needed.
- This is a water-hygiene page. It covers the organism and its control in a water system. Anything clinical is a matter for the NHS or a clinician.
Pseudomonas aeruginosa is a waterborne bacterium that colonises the wet end of a water system: the tap, its flow straightener, the spout and the drain beneath it. It lives in biofilm, the thin slime layer that forms on any surface in constant contact with water, and from there it can contaminate the water drawn from that outlet.
It is an opportunistic organism, which means it rarely troubles healthy people but poses a genuine risk to those who are seriously unwell. In water-hygiene terms it matters most in healthcare, and the way it behaves in the system is quite different from legionella. This page explains where pseudomonas comes from in a water system and how it is controlled. It is not a medical page: for symptoms, diagnosis or treatment, speak to the NHS or a clinician.
Where pseudomonas lives in a water system
Pseudomonas is common in wet environments, so the question is not usually whether it is present nearby but whether it has established itself in the parts of the system that feed an outlet. It favours the very last components before water leaves the tap: the flow straightener (the small mesh insert in the spout, sometimes called an aerator), the spout itself, the thermostatic mixing valve that blends hot and cold, and the seals and washers inside the tap assembly.
It also thrives around sinks. Sink traps, waste outlets and drains hold standing water and organic matter, and a tap that discharges straight into the drain can throw contaminated droplets back up onto the spout and the surrounding area. Poorly designed hand-wash basins, where the stream lands directly over the waste, are a recognised route for this kind of contamination. Once biofilm is established in any of these places, water passing through picks up the organism on its way out.
Why augmented care is the main concern
The dedicated UK guidance is HTM 04-01 Part C, which deals specifically with pseudomonas in augmented care. Augmented care covers areas where patients are especially vulnerable to infection: neonatal units, intensive care, units treating severe burns, and those caring for transplant or immunocompromised patients. In these settings water that would be harmless to most people can present a real hazard, and the outlets closest to patient care receive particular attention.
Managing this is usually the job of a Water Safety Group, working from a written Water Safety Plan. The focus is practical: knowing which outlets serve high-risk patients, keeping those outlets clean and in regular use, and acting quickly when a sample suggests colonisation. Outside healthcare, pseudomonas is far less commonly a priority, though it can still be relevant anywhere vulnerable people are cared for. Pseudomonas is not the only outlet organism watched in these settings; stenotrophomonas maltophilia is another waterborne opportunistic pathogen managed along much the same lines.
How pseudomonas differs from legionella
This is the point that trips people up. Legionella tends to grow throughout a system wherever water sits still in the growth range of roughly 20°C to 45°C, and it is controlled system-wide by keeping hot water hot, cold water cold, and stagnation to a minimum. It causes disease mainly when contaminated water is breathed in as a fine aerosol.
Pseudomonas behaves differently. It concentrates in the biofilm at the outlet, in the last few metres of pipe and in the tap fitting itself, rather than colonising the whole network. That changes where control effort goes. Temperature control of the wider system still matters, but it does not reach the very end of the run where the tap has cooled towards room temperature, and that is exactly where pseudomonas settles. So control leans heavily on the outlet: the fitting, the flow straightener, the drain and how often the tap is used.
Because pseudomonas colonises the tap and its fittings, the last few metres of the system and the outlet itself matter more than the temperature deep in the pipework.
Controlling pseudomonas at the outlet
Control is a combination of measures, and no single one is enough on its own. The recognised approaches include:
- Reducing stagnation. Outlets that are used every day turn their water over and give biofilm less chance to establish. Little-used taps should be flushed on a set schedule or removed. Our guide on how long to run taps when flushing covers the same principle.
- Cleaning and disinfecting outlets. Taps, spouts and their fittings are cleaned and disinfected on a planned basis, with the method and frequency set by the risk in that area.
- Managing flow straighteners and TMVs. Flow straighteners trap debris and harbour biofilm, so many high-risk areas descale, disinfect or simply remove them, and TMV assemblies are maintained as part of the same routine.
- Avoiding splashing from drains. Basins and taps are positioned and used so the stream does not fall directly onto the waste, reducing the chance of contaminated droplets reaching the spout.
- Point-of-use filters. Where an outlet serves very high-risk patients, a point-of-use filter fitted at the tap removes bacteria from the water at the point it is drawn. These are a control at the outlet, not a fix for the wider system, and they are changed on a defined interval.
Persistent contamination that survives cleaning may mean a component, or the whole tap, has to be replaced. Decisions of that kind, and the design of any remedial work, are best taken with a competent water-hygiene specialist.
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.
Sampling and monitoring
Pseudomonas is looked for by sampling water from outlets, not from the middle of the system. In augmented care that means the taps and TMV assemblies serving high-risk patients. Samples are usually 100 millilitres and are often taken both before and after flushing, so the result shows whether contamination sits in the fitting itself or further back. Results are reported as colony-forming units per 100 millilitres, and any detection in these areas prompts investigation into where the organism has established and what needs to change.
Sampling is a snapshot, so it sits alongside the day-to-day controls rather than replacing them. The general principles of taking and interpreting water samples are covered in our page on legionella testing, and the outlet focus is the same reason shower heads and spray outlets receive close attention for legionella.
Where this fits in the wider guidance
Pseudomonas control does not replace legionella control; it runs alongside it. A building following HSG274 and the duties in ACOP L8 will already be managing temperature and stagnation, and in healthcare HTM 04-01 Part C adds the outlet-focused measures above for augmented care. Whatever the setting, the cleaning, flushing and sampling you carry out should be written down. Keeping a clear record in a log book is how you show what was done, when, and to which outlets, and it feeds into your overall water-hygiene compliance. If a sample flags a problem, the same disciplined response applies as when legionella is detected: investigate, act on the source, and confirm the control has worked.
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 the sources of pseudomonas aeruginosa in water?
The organism is common in wet environments, so the sources in a water system are the places where it can settle and multiply near an outlet. These include the flow straightener in the spout, the tap spout and its internal seals, thermostatic mixing valves, and the sink traps and drains below the basin.
It spreads through biofilm at these points, through droplets splashed up from a contaminated drain, through infrequently used outlets where water stands still, and through contaminated components or fittings introduced during maintenance. Because it favours the wet end of the system, the outlet itself is usually the source rather than the pipework deep in the building.
How do you remove pseudomonas from a water system?
In practice you control it rather than remove it entirely, because the organism is so widespread in wet environments. The recognised measures are cleaning and disinfecting outlets, descaling or removing flow straighteners, keeping taps in regular use to reduce stagnation, maintaining or replacing contaminated components, and fitting point-of-use filters at very high-risk outlets.
Contamination that survives repeated cleaning may mean the fitting or the whole tap has to be replaced. Decisions on remedial work, and any system-wide disinfection, are best taken with a competent water-hygiene specialist, and in healthcare through the Water Safety Group.
How is pseudomonas different from legionella?
Legionella tends to grow throughout a system wherever water stagnates in the range of roughly 20°C to 45°C, and it is controlled system-wide through temperature and stagnation control. It causes disease mainly when contaminated water is breathed in as a fine aerosol.
Pseudomonas concentrates in the biofilm at the outlet, in the last few metres of pipe and in the tap fitting itself, rather than colonising the whole network. That is why its control focuses on the fitting, the drain and how often the tap is used, and why the two organisms are sampled and managed in different ways.
Where is pseudomonas tested for in water systems?
Pseudomonas is sampled at outlets rather than from the body of the system, and in healthcare the focus is augmented care: the taps and thermostatic mixing valve assemblies serving the most vulnerable patients. Samples are usually 100 millilitres and are often taken both before and after flushing, so the result shows whether contamination sits in the fitting or further back.
Results are reported as colony-forming units per 100 millilitres, and sampling under HTM 04-01 Part C concentrates on the outlets closest to high-risk patients rather than across the whole building. Any detection in these areas prompts investigation into the source and the controls in place.
Related water hygiene products and services from trusted UK providers will appear here.