Legionella rarely multiplies on its own in a building’s water. It does most of its growing inside something else: the single-celled amoebae that graze the biofilm coating your pipe walls. Picture those amoebae as the bacterium’s nursery, and a lot of stubborn control problems start to make sense.

If you already know the temperature bands and the stagnation rules, this is the layer underneath them. It is the reason a system that looks controlled on paper can still hand you a positive sample, and the reason a textbook disinfection sometimes does not stick.

The partnership underneath the biofilm

In rivers and soil, Legionella lives as a parasite of free-living protozoa — it infects amoebae and grows inside them. The same trick works inside plumbing. Amoebae crawl over the biofilm grazing on bacteria; they engulf Legionella expecting a meal; Legionella resists being digested, hijacks the cell, and replicates inside it. One host can release many bacteria when it eventually bursts.

This is why biofilm is not just dirt to be hosed off. The slimy film on a tank wall or inside a flexible shower hose is habitat: a food supply, a surface to settle on, and a population of amoebae to hide inside. The CDC describes biofilm, warm temperatures, low disinfectant levels and slow or stagnant water as the conditions that let Legionella grow [1]. Add amoebae to that picture and the bacterium gains both an incubator and a bodyguard.

Picture what is happening inside the pipe

Most articles would put a diagram here. If you cannot see one, sketch it yourself — it is worth the thirty seconds, because the layout explains nearly every control decision that follows.

Imagine a short section of pipe wall in cross-section, building up in layers:

  1. The surface. No pipe stays clean. Within days a thin biofilm of bacteria, scale and organic matter forms across the wetted surface.
  2. The grazers. Free-living amoebae (genera such as Acanthamoeba and Vermamoeba) live on and in that film, feeding on the bacteria within it.
  3. The takeover. Legionella cells sit in the biofilm. An amoeba engulfs them, but instead of digesting them it becomes a host: Legionella multiplies inside, sheltered from whatever is happening in the water flowing past.
  4. The release. The host eventually ruptures, sending a pulse of Legionella into the bulk water — some of it packaged inside vesicles that disinfectant struggles to penetrate.
  5. The escape hatch. Under stress — a heat flush, a dose of biocide, a system left to dry — some amoebae encyst, forming a tough wall. Any Legionella inside ride out the stress protected.
  6. The exposure path. The released bacteria travel with the bulk water to an outlet. A shower or spray tap turns them into a fine aerosol, and someone inhales it.

Read the arrows in one line: source water feeds the biofilm habitat, the amoebal host amplifies the bacteria, a release seeds the bulk water, an aerosol-forming outlet delivers it to the lungs. The two sidesteps — vesicles and cysts — are the bits that frustrate treatment.

Why this changes what “control” means on site

Most controls act on the water column. A heat disinfection or a chlorination targets the free-swimming bacteria moving through the system. Legionella locked inside an amoeba, a vesicle or a cyst sits behind a wall that ordinary water-side measures may not breach. So a treatment that “should have worked” can leave a seeded biofilm intact, and counts rebound once conditions ease.

As a general expectation, guidance puts active growth in roughly the 20–45°C band and progressive kill-off above about 60°C [2], but those figures describe what happens to bacteria the heat actually reaches. Confirm the exact targets and how to apply them through your assessment — and remember that a protected reservoir on the pipe wall changes how much faith a single reading deserves.

It also reframes what a clean sample tells you. A negative result describes the water at one outlet at one moment; it says little about bacteria still bound up in hosts on the wall. That is not a reason to distrust sampling — it is a reason to read it as one input, not a verdict.

The practical conclusion is blunt: attack the habitat, not only the chemistry and the temperature. Removing biofilm and eliminating stagnation removes the nursery. Dosing harder while a dead leg quietly grows slime is fighting the symptom.

What it does not change

There is no separate amoebae-control regime to run, and no test for amoebae that a duty holder is expected to add. You do not manage the protozoa directly. You manage the conditions they depend on, which are the familiar ones: keep hot water hot, keep cold water cold, keep water moving, and keep surfaces and fittings clean, because those same measures starve and disturb the biofilm the amoebae live in [3]. The amoebal story is not an argument to over-engineer; it is an argument to take dead legs, scale and neglected fittings as seriously as you take the thermometer.

This is general guidance, not a diagnosis or a design specification. The amoebal mechanism explains why controls sometimes underperform; it does not set your temperatures, your dosing or your sampling intervals — those come from a competent, site-specific risk assessment of your actual system [3]. If a sample comes back positive after a disinfection that looked faultless, treat it as a question about surviving habitat rather than a lab error, and get a competent person to find where the reservoir is hiding. Temperature and stagnation are unpacked further in Temperature and Legionella growth, and the wider species picture in Legionella species and serogroups beyond L. pneumophila.

Common questions

Do I need to test my water specifically for amoebae?

Generally no. Routine programmes target Legionella and the conditions that let it grow, and amoebae are not a standard duty-holder test. The useful move is controlling the biofilm they live in. What you sample for, and how often, follows the risk assessment rather than a fixed shopping list [4].

Why did Legionella come back after we disinfected?

A common reason is that bacteria sheltered inside amoebae or biofilm on the pipe wall survived a treatment aimed at the water column, then reseeded the system. Recurrence usually points to surviving habitat — a dead leg, scale, a low-flow branch, a slime-lined hose — rather than a failed chemical. Hunt the reservoir before you repeat the dose.

Does this mean my temperature regime is pointless?

No. Temperature control still suppresses growth and remains a core control. The amoebae point is that heat and biocide do their best work when biofilm and stagnation are managed too, so the treatment actually reaches the bacteria instead of leaving a protected pocket behind.

Where to start this week

Pick your single worst stagnation point — the dead leg, the shower nobody uses, the oversized cold tank sitting in a warm plant room — and look at it as a biofilm problem, not just a temperature one. Get eyes on the fitting itself: scale around the head, sediment in the base, a flexible hose or aerator harbouring slime. Write down why that point is your priority and what you did about it. Recording the reasoning, not just the task, is what turns the amoebae science into a control decision you can defend. If you want the bigger frame around that habit, Beyond compliance: striving for Legionella control excellence covers control that goes past minimum compliance.

Sources

[1] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems — Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm