What is the difference between legionella and pseudomonas?
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Key points
- Legionella infects by inhalation of contaminated aerosol; pseudomonas infects by contact at the outlet, in already-vulnerable patients.
- Legionella is a whole-system problem managed by temperature; pseudomonas is largely an outlet-level problem managed by cleaning, flushing, and sampling.
- Legionella control sits under ACOP L8 and HSG274; healthcare pseudomonas control sits under HTM 04-01 Part C.
- The shared enemy is biofilm, which shelters both organisms and blunts both heat and disinfectant.
- A system can be well controlled for legionella and still grow pseudomonas at a tap, which is why augmented care adds outlet sampling on top of temperature control.
Legionella and Pseudomonas aeruginosa are both waterborne bacteria, both live in biofilm, and both can cause serious infection, but they get into people by different routes and they are controlled in different ways. Legionella is inhaled in contaminated water droplets and causes pneumonia; pseudomonas reaches patients through direct contact with contaminated water at taps, showers, and drains, and it threatens people who are already severely ill.
The comparison matters because the control regime follows the route of infection. A building managed only for legionella, on temperature alone, can still harbour pseudomonas at its outlets. In most premises that is a minor concern; in augmented care it is a central one. The two organisms are covered separately in what is legionella and pseudomonas in water systems; this page is about the practical difference.
The two organisms compared
| Legionella | Pseudomonas aeruginosa | |
|---|---|---|
| Disease caused | Legionnaires' disease, a severe pneumonia; also Pontiac fever | A range of healthcare infections: respiratory, wound, urinary, bloodstream |
| Route of infection | Inhaling contaminated aerosol from showers, spray taps, cooling towers, spa pools | Direct contact with contaminated water or splashes at outlets, and aspiration in vulnerable patients |
| Who is most at risk | Older people, smokers, people with lung disease or weakened immunity | Severely immunosuppressed patients, burns patients, those on invasive devices: the augmented care population |
| Where it lives in the system | Throughout hot and cold systems where water sits at 20 to 45°C: tanks, calorifiers, long pipe runs, dead legs | Concentrated at the point of use: tap spouts, flow straighteners, shower heads and hoses, and drain traps |
| Primary control | Temperature: hot stored at 60°C, delivered at 50°C or more, cold below 20°C, plus flushing and cleaning | Outlet hygiene: cleaning and descaling, flushing little-used outlets, outlet sampling in augmented care, point-of-use filters as an interim measure |
| Governing guidance | ACOP L8 and HSG274 for all premises | HTM 04-01 Part C in healthcare, on top of the general legionella framework |
Different routes, different controls
Legionella does its harm in the air. The bacterium multiplies in tepid, stagnant water anywhere in the system, and infection happens when that water is turned into a fine spray and breathed deep into the lungs. That is why the control philosophy is systemic: keep the whole hot system too hot and the whole cold system too cold for growth, turn over stagnant water by flushing, and remove the dead ends where it collects. The temperature chart is, in effect, the legionella strategy on one page.
Pseudomonas does its harm at the point of contact. It establishes in the biofilm of the last metre: inside the tap spout, on the aerator, in the shower hose, in the U-bend. From there it contaminates water used for washing patients, rinsing equipment, or cleaning wounds. Temperature helps, pseudomonas dislikes hot water, but the last metre of any outlet is at room temperature by definition, so outlet-level hygiene carries the load. In augmented care, HTM 04-01 adds routine outlet sampling, and a point-of-use filter is a recognised interim barrier while a contaminated outlet is dealt with.
Legionella is controlled in the plant room and the pipe runs; pseudomonas is controlled at the tap. A building can pass every temperature check and still grow pseudomonas in a neglected outlet.
Who should worry about which
For most duty holders, landlords, offices, hotels, schools, legionella is the organism that matters and pseudomonas is background. The people in those buildings are generally healthy, contact with outlet water does them no harm, and the aerosol risk from showers and spray taps is the live one. The written scheme of control, built on temperature and flushing, addresses it.
In healthcare the picture inverts for the highest-risk areas. Augmented care units, intensive care, neonatal, transplant, haematology and oncology wards, house exactly the patients pseudomonas threatens, and outbreaks there have driven some of the most serious water-safety incidents in the NHS. That is why healthcare adds the pseudomonas layer on top of the legionella regime rather than treating one control programme as covering both. Other waterborne opportunists behave similarly, as our page on stenotrophomonas maltophilia describes.
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.
The common ground: biofilm
Both organisms share one refuge. Biofilm, the slime layer that forms on wet surfaces, shelters bacteria from heat and disinfectant and supplies them with nutrients, which is why persistent problems with either organism usually trace back to fouled outlets, scale, sediment, or stagnant branches rather than to bad luck. The practical disciplines that strip biofilm out, descaling shower heads, flushing little-used outlets, keeping tanks clean and sealed, are therefore doing double duty, whatever organism your setting worries about most. Record them all, in the same log book, kept for at least five years.
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 is the difference between legionella and pseudomonas?
They are different bacteria that threaten people in different ways. Legionella causes Legionnaires' disease when contaminated water droplets are inhaled deep into the lungs, so the risk is aerosol from showers, spray taps, and cooling towers, and the main control is keeping hot water hot and cold water cold. Pseudomonas aeruginosa mainly harms severely ill or immunosuppressed patients through direct contact with contaminated water at taps and outlets, so the risk concentrates at the outlet itself and is managed under healthcare guidance, HTM 04-01 Part C.
Is pseudomonas more dangerous than legionella?
They are dangerous to different groups. Legionella can cause severe pneumonia in older people, smokers, and anyone with weakened immunity, and it can affect anyone exposed to a heavily contaminated aerosol. Pseudomonas aeruginosa rarely harms healthy people but is a serious threat in augmented care units, where patients are severely immunosuppressed, have breached skin barriers, or rely on invasive devices. For a hospital ward the pseudomonas risk may dominate; for a hotel or office, legionella is the primary concern.
Does controlling legionella also control pseudomonas?
Only partly. Temperature control, the backbone of legionella management, helps because pseudomonas dislikes hot water, but pseudomonas colonises the last metre of pipework, the tap, and the drain, where water is at room temperature and biofilm forms. Controlling it therefore depends on outlet-level measures: regular cleaning and descaling of taps and shower heads, flushing little-used outlets, replacing faulty flow straighteners, and, in augmented care, routine outlet sampling and point-of-use filters where needed.
Where does pseudomonas come from in a water system?
Pseudomonas aeruginosa arrives in trace amounts in the incoming supply and establishes itself in biofilm at outlets: inside tap spouts, on flow straighteners and aerators, in shower heads and hoses, and in the wet traps of drains. It can also be seeded backwards from drains and splashing. Because it lives at the point of use rather than in the main pipework, a building can have a well-controlled hot and cold system and still find pseudomonas at an individual tap.
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