How do amoebae help legionella grow?
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Key points
- Free-living amoebae graze on biofilm and engulf legionella without digesting it. The bacteria multiply inside the amoeba and are released in greater numbers.
- The host relationship is legionella's amplification engine: serious growth in a building system happens inside amoebae, not in open water.
- Inside a host, and even more inside a cyst, legionella is shielded from heat and chlorine, which explains clear-then-recolonised results after disinfection.
- Amoebae live wherever biofilm grows, so controlling biofilm controls the whole food chain.
- Movement, correct temperatures and physical cleaning work precisely because they starve this relationship of its habitat.
Legionella does most of its growing inside another organism. Free-living amoebae, microscopic single-celled creatures that graze on bacteria in biofilm, take up legionella as food but cannot digest it. The bacteria survive inside the amoeba, multiply there, and are eventually released back into the water in far greater numbers than were taken in. Understanding this host relationship explains almost everything about how legionella behaves in a building, and why the standard controls are what they are.
How the relationship works
Amoebae such as Acanthamoeba and Vermamoeba live on wet surfaces wherever biofilm grows, because biofilm is their grazing ground. An amoeba feeds by engulfing bacteria and digesting them. Legionella has evolved to interrupt that process: once engulfed, it blocks the digestion machinery, settles into a compartment inside the amoeba, and uses the host's own resources to multiply. When the host cell dies or is stressed, the bacteria are released, sometimes packed in protective vesicles, ready to infect the next amoeba or be carried off in the water flow.
This is why legionella growth in a real building is better understood as a food chain than as a single bacterium drifting in water. Biofilm feeds the amoebae; the amoebae incubate the legionella; the water flow carries the amplified bacteria to the outlet. A trace number arriving in the mains supply becomes a colonised system through this cycle, in the warm, stagnant corners where biofilm establishes.
Why hosts make legionella hard to kill
The host relationship also explains legionella's resilience. A bacterium inside an amoeba is physically shielded from disinfectant in the surrounding water, and some amoebae go further: when conditions turn hostile they form cysts, tough dormant shells that protect everything inside them, including any legionella they are carrying, from heat and chemical treatment.
This is the mechanism behind a pattern every water hygiene practitioner recognises. A system is dosed with chlorine, the follow-up sample comes back clear, and a few weeks later the count is back up. The treatment killed the free-floating bacteria it could reach; the population sheltering inside hosts in the biofilm survived and re-seeded the system. A clear sample straight after disinfection is a snapshot, not a cure, and our page on legionella in water explains how the action levels should be read with that in mind.
Legionella's real reservoir is not the water but the biofilm's food chain. Kill the free bacteria and leave the hosts intact, and the system re-seeds itself.
What this means for control
The host relationship is the reason the standard control regime is built the way it is. Each measure attacks the food chain at a different point:
- Temperature. Keeping cold water below 20°C and hot water at 50°C or more at outlets, with storage at 60°C, holds the system outside the band where biofilm, amoebae and legionella all flourish. Our temperature checker records whether the targets are being met, and the figures are set out in temperature control.
- Movement. Stagnant water is where biofilm establishes and the grazing begins, so flushing little-used outlets and removing dead legs breaks the cycle at its starting point.
- Physical cleaning. Because hosts and bacteria shelter inside biofilm and behind scale, physically cleaning tanks and descaling outlets removes the habitat that chemical treatment cannot reliably penetrate.
- Disinfection as a support. Chlorination still has its place, but it works best after the physical removal, when the shielded population has been stripped away and the chemical can reach what remains.
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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The takeaway for duty holders
You do not need to identify or count amoebae, and routine monitoring does not try to. The practical lesson is strategic: legionella control is habitat control. A system that is kept moving, kept at the right temperatures and kept physically clean starves the amoebae of their grazing ground, and without hosts legionella cannot build the numbers that make a system dangerous. That is why the guidance in HSG274 and the duty under ACOP L8 keep returning to the same trio of temperature, turnover and cleanliness, and why the record of those controls in your log book is the evidence the regime is holding.
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 eats legionella in a water system?
Free-living amoebae graze on bacteria in biofilm and take up legionella along with everything else, but they cannot digest it. Instead of being destroyed, legionella survives inside the amoeba, multiplies there, and is eventually released back into the water in far greater numbers. So the organism that appears to eat legionella is actually its incubator, which is why amoebae sit at the centre of how legionella behaves in building water systems.
Can legionella survive without amoebae?
Legionella can persist in water and biofilm on its own, but it multiplies poorly outside a host. The serious amplification that turns trace background numbers into a colonised system happens inside amoebae and similar protozoa. This is why controlling the biofilm that feeds the amoebae is such an effective lever: remove the grazing ground and you remove the hosts, and without hosts legionella cannot build dangerous numbers.
Do amoebae protect legionella from chlorine?
Yes. A legionella cell inside an amoeba is shielded from disinfectant in the water, and some amoebae can form tough cysts that protect their contents from heat and chemical treatment even more effectively. This is one reason a system can test clear straight after disinfection and be colonised again weeks later: the free bacteria were killed, but the population sheltering inside hosts in the biofilm survived and re-seeded the system.
Where do amoebae live in a water system?
Amoebae live wherever biofilm grows, because biofilm is their food: on pipe walls, on tank surfaces, in sediment at the base of tanks and calorifiers, around tap washers and inside shower heads and hoses. They favour the same warm, stagnant conditions as the bacteria they graze on, so dead legs, little-used outlets and oversized storage are where the whole food chain establishes.
Why does the amoeba relationship matter for legionella control?
It explains why the standard controls work and why shortcuts fail. Keeping water below 20°C or above 50°C, keeping it moving, and physically removing biofilm and scale all attack the food chain legionella depends on. It also explains why a one-off chlorine dose is not a lasting fix: it kills the bacteria it can reach but leaves the sheltered hosts to repopulate the system. Control the habitat and you control everything living in it.
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