Which bacteria live in hot tubs?
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Key points
- Hot tubs carry more than legionella. The main organisms are legionella, Pseudomonas aeruginosa and non-tuberculous mycobacteria, all at home in warm, aerated water.
- Pseudomonas causes hot tub rash and ear infections; it is the classic sign that a tub's disinfection has failed.
- Mycobacteria tolerate chlorine and heat better than legionella and are linked in medical literature to a lung inflammation known as hot tub lung.
- Biofilm in the pipework, jets and filters shelters all of them, which is why a chlorine reading alone never tells the whole story.
- The dedicated HSE guidance, HSG282, is titled the control of legionella and other infectious agents precisely because the risk is wider than one species.
Hot tubs can harbour several bacteria besides legionella, most importantly Pseudomonas aeruginosa, which causes hot tub rash and ear infections, and non-tuberculous mycobacteria, which are linked to lung inflammation. All of them thrive in warm, aerated water and shelter in biofilm, and all are managed by the same HSG282 control regime.
Why a hot tub suits bacteria so well
Everything that makes a hot tub pleasant makes it biological. The water sits at 30 to 40°C, which is the middle of the temperature band where waterborne bacteria multiply fastest. The jets keep it aerated and circulating through metres of warm pipework. Bathers continuously add sweat, skin cells, cosmetics and other organic matter, which feeds bacteria and consumes disinfectant at the same time. And the whole system, the pipes, the jets, the filter, the balance tank, offers endless wet surface for biofilm to colonise.
Our page on legionella in hot tubs and spa pools covers the legionella side of this in detail. This page is about the rest of the cast, because legionella has never been the whole story. The HSE's dedicated guidance for these systems, HSG282, is deliberately titled the control of legionella and other infectious agents in spa-pool systems.
The bacteria you are actually managing
| Organism | What it is known for | What its presence tells you |
|---|---|---|
| Legionella (several species) | Legionnaires' disease and Pontiac fever from inhaled aerosol | Warm, under-disinfected, often stagnant water; the most serious outcome a tub can produce |
| Pseudomonas aeruginosa | Hot tub rash (folliculitis) and ear infections | Disinfectant residual has fallen away; pseudomonas thrives exactly when chlorine or bromine drops |
| Non-tuberculous mycobacteria | Lung inflammation (hot tub lung) reported in medical literature | Established biofilm; their waxy cell wall shrugs off chlorine levels that clear other bacteria |
| Other opportunists (for example stenotrophomonas) | Mainly a risk to vulnerable or immunocompromised people | A general loss of microbiological control in the system |
HSE HSG282; UKHSA public health guidance; published clinical literature on recreational-water outbreaks.
Pseudomonas and hot tub rash
Pseudomonas aeruginosa is the organism behind hot tub rash, properly called pseudomonas folliculitis. It shows up as an itchy rash, sometimes with small pus-filled spots around hair follicles, appearing within a day or two of using a contaminated tub, often worst where a swimming costume held the water against the skin. The same organism causes outer-ear infections, which is why some bathers leave a badly managed tub with sore ears as well as sore skin.
Two things make pseudomonas useful to know about even if you only care about legionella. First, it multiplies fast when disinfectant residuals drop, so a pseudomonas problem is direct evidence that the tub's chemistry has failed. Second, it lives in the same biofilm as legionella, so a tub that grows one is offering a home to the other. A rash among bathers is not a minor customer complaint; it is a water-quality report you did not have to pay for.
Mycobacteria and hot tub lung
Non-tuberculous mycobacteria are environmental organisms that colonise the same warm, wet surfaces as legionella, but with an important difference: their waxy cell wall makes them markedly more resistant to chlorine and to heat. A disinfectant level that keeps coliforms and pseudomonas in check cannot be assumed to clear an established mycobacterial population, particularly inside biofilm.
The medical literature describes a condition known as hot tub lung, a lung inflammation (a form of hypersensitivity pneumonitis) seen in people repeatedly exposed to aerosol from tubs contaminated with mycobacteria. It is uncommon, but it underlines the same point from a different direction: the aerosol a hot tub produces contains whatever is living in its water and its pipework. This page describes the water-system side only; anything clinical belongs with a doctor.
Biofilm: the shared habitat
None of these organisms needs to live in the open water at all. They live in biofilm, the slime layer that builds on every wet surface in the system: inside the pipework, around the jets, in the filter and in the balance tank. Biofilm feeds bacteria, shields them from disinfectant, and sheds them back into the water in pulses, which is why a tub can test clear one day and turn cloudy or cause a rash the next.
This is the mechanical reason HSG282 does not rely on chemistry alone. Disinfectant handles the open water; dilution, filter backwashing, physical cleaning and periodic drain-downs handle the surfaces. A regime that only doses and never drains is fighting half the battle.
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 control regime that covers all of them
The good news is that one regime controls the whole cast, because they all depend on the same conditions. HSG282 sets it out for commercial spa pools, and the same logic applies to any hot tub:
- Continuous disinfection with chlorine or bromine held at the correct residual, typically 3 to 5 mg/l free chlorine for a commercial spa pool, with pH held around 7.0 to 7.6 so the disinfectant stays effective.
- Filtration and circulation so contamination is physically removed and disinfectant reaches every part of the system.
- Dilution, replacing a proportion of the water with fresh mains water in step with bather load, to flush out what the disinfectant cannot remove.
- Periodic drain-down and cleaning of the whole system, including pipework and balance tank, to strip out biofilm and debris.
- Recorded daily checks of disinfectant and pH, several times a day during use, plus routine microbiological sampling to verify the chemistry is genuinely working.
If the tub is provided for guests, members or the public, including at a holiday let, this regime is a legal duty under the Health and Safety at Work etc. Act framework, not a recommendation. A private household tub sits outside HSG282, but the organisms do not know the difference, and the same routine is what keeps a domestic tub safe to sit in.
A hot tub does not have a legionella problem or a pseudomonas problem or a mycobacteria problem. It has one warm, aerated, under-managed water problem, and the organisms take turns.
What an illness cluster tells you
Because these organisms share a habitat, illness among bathers is diagnostic of the system rather than just the individual. A cluster of rashes points at failed disinfection. A flu-like illness cluster points at aerosolised legionella, as in the Lochgoilhead outbreak, where a whirlpool spa produced a Pontiac-fever-type illness in a large share of the people exposed. Either way, the response is the same: take the tub out of use, investigate the water and the regime, and put right the failure before anyone else sits in it. Recording the regime day by day, in a log book, is what lets you show an inspector, or a court, that the controls were real rather than intended.
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 bacteria grow in hot tubs?
The main ones are legionella, which causes Legionnaires' disease and Pontiac fever; Pseudomonas aeruginosa, which causes hot tub rash and ear infections; and non-tuberculous mycobacteria, hardy environmental organisms linked to lung irritation. All of them thrive in warm, aerated water and shelter in the biofilm that lines the pipework, jets and filters, which is exactly the environment a hot tub provides.
What is hot tub rash?
Hot tub rash is the common name for pseudomonas folliculitis, an itchy skin rash that appears after sitting in water contaminated with Pseudomonas aeruginosa. The bacteria infect hair follicles, often where swimwear has held contaminated water against the skin, and the rash usually clears on its own within days. It is a reliable sign that the disinfection in that tub has failed. Anyone with a severe or persistent rash should see a GP; this page is about the water system, not medical advice.
Can a hot tub cause a lung infection?
Yes, in two ways. Legionella carried in the aerosol from jets and bubbles can cause Legionnaires' disease or the milder Pontiac fever when inhaled. Medical literature also describes hot tub lung, a lung inflammation linked to breathing in aerosol containing mycobacteria from contaminated tubs. Both risks come from the same failure: warm, under-disinfected water being turned into a breathable mist.
Does chlorine kill all the bacteria in a hot tub?
Only while the right free residual is actually present in the water, which for a commercial spa pool means roughly 3 to 5 mg/l at a controlled pH of about 7.0 to 7.6. Warm aerated water and bather pollution consume chlorine fast, organisms inside biofilm in the pipework are shielded from it, and mycobacteria are unusually chlorine-tolerant even in open water. Disinfection has to be continuous and paired with dilution, cleaning and periodic drain-downs.
How do you keep hot tub water safe from bacteria?
Follow the HSG282 regime: continuous disinfection with bromine or chlorine held at the correct residual, filtration and circulation, checks of disinfectant and pH several times a day recorded in a log, regular replacement of a proportion of the water, periodic full drain-down and cleaning of the whole system, and routine microbiological sampling to verify the chemistry is working. If the tub is provided for guests or the public, this is a legal duty, not merely good practice.
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