Legionella does not really live in your water. It lives on the walls of the things that carry your water — pipes, tanks, shower hoses, washers, the inside of a thermostatic mixing valve — held there in a thin, slippery layer called biofilm. The water is just how it travels, and how it reaches a pair of lungs. Grasp that one shift and a lot of confusing advice starts to make sense.

It explains the most frustrating thing duty holders run into. A water sample comes back clear, then the same system trips an assessment weeks later. The sample measured the water. The problem was on the pipe wall. Biofilm is where Legionella shelters, feeds, and survives the very controls — heat and disinfectant — that are meant to remove it.

What a biofilm actually is

Leave any wetted surface alone and bacteria will colonise it. They attach, then secrete a sticky matrix of sugars and proteins that glues the colony to the surface and to itself. Within days you have a living film: bacteria, sediment, scale and the single-celled organisms that graze on them, all bound up in a gel you cannot see and rarely can feel. Every plumbing system has some. The question is never whether you have biofilm. It is how much, where, and what is living in it.

Legionella is an opportunist in that community. It rarely builds a film on its own, but it thrives inside one built by others, and it can shelter inside the protozoa that feed on the film — a bacterium tucked inside a host, inside a film, inside a pipe. Each layer is another barrier between the organism and your control measures. The CDC lists biofilm, warm temperatures, stagnant water and low disinfectant levels as the conditions that let Legionella multiply [1]. Biofilm is the thread running through all four: it forms fastest in warm, still water, it harbours the bacteria, and it physically shields them from chlorine and other treatments.

Where it takes hold in a real building

Biofilm is not spread evenly. It concentrates wherever water is warm, slow, and in contact with a rough or complex surface — which in most buildings means a short list of usual suspects.

The flexible shower hose is the classic. It is warm from the last use, ridged on the inside, usually left full of standing water, and it ends in a head that breaks that water into exactly the fine aerosol Legionella needs to reach someone. A hose on a rarely-used staff shower can hold a thriving film while the rest of the system looks healthy.

Dead legs and capped-off branches are biofilm factories. Water sits there permanently, undisturbed, drifting to room temperature — and because the branch still joins the live system, it quietly seeds bacteria back into water that does move. Tracing and physically removing redundant pipework does more for biofilm control than almost anything you can dose into the water.

Then there is sediment. Scale, rust and debris settle into the base of cold water tanks and the bottom of calorifiers, and that sludge is both food and shelter. A calorifier that is never drained down and inspected can deliver a perfectly correct outlet temperature while harbouring a warm, well-fed reservoir at its base.

Components matter too. Thermostatic mixing valves blend hot and cold to a safe delivery temperature right at the point of use, which means they create a pocket of lukewarm water inside a fiddly little assembly of seals and strainers. Good for scald protection, awkward for biofilm. TMVs earn their place, but they need cleaning and descaling on a schedule, not fitting and forgetting.

Why you cannot simply kill it

Here is the uncomfortable part, and the bit generic guidance skips. You do not eradicate biofilm. You manage it.

Keeping hot water hot and cold water cold suppresses growth and is the backbone of control, but heat reaches the free-floating bacteria in the water far more easily than the ones embedded in a film on the pipe wall [2]. A one-off chemical disinfection knocks the population right back, and then biofilm regrows from whatever survived in the corners the treatment never quite reached. That is why control has to be a continuous regime rather than an event, and why HSE frames it as a managed system of temperature, water movement, cleanliness and monitoring instead of a single fix [3][4].

In my view the most common mistake is treating a disinfection certificate as a finish line. It is a reset, not a cure. The clock on regrowth starts the day after the contractor leaves.

A biofilm walk-round you can actually do

You will not see biofilm directly without dismantling fittings, but you can find the conditions that breed it. Walk the building with a notepad and look for the places it hides. Group your notes so the gaps stand out.

Find the warm, still spots

  • Identify every outlet used less than roughly once a week — staff showers, spare bathrooms, cleaners’ sinks, the tap behind the bar.
  • List the flexible shower hoses, and note the date each head was last cleaned or replaced.
  • Flag any run that feels lukewarm where it should be cold, or that has dropped below hot at a far outlet.

Hunt the reservoirs

  • Trace the pipework for dead legs and capped branches left over from old layouts or removed equipment.
  • Check cold water tanks for sediment, debris and a properly sealing lid; record the date of the last inspection.
  • Confirm calorifiers have been drained down and the base inspected, not just monitored at the outlet.

Record the decision, not just the reading

  • For each higher-risk fitting, write down why its control exists and what happens when a check fails.
  • Log cleaning, descaling and flushing with dates, so regrowth between visits is actually visible.

Anything you cannot answer is a gap to feed into the risk assessment, not a box to tick blank.

Where to start

None of this replaces a competent, site-specific risk assessment. The right temperatures, the cleaning intervals, and whether to disinfect or sample for your particular building depend on its layout, its users and how it is run — a generic checklist cannot set those numbers for you. Use this to ask sharper questions, then let a competent person and your written scheme supply the answers.

With that said, pick the single worst offender first. In most buildings that is a warm, rarely-used shower on a flexible hose, so start there: clean or replace the head and hose, get it onto a flushing schedule, and write down what you did. Then book the calorifier and cold tank inspections you have been deferring. Biofilm rewards deferral, which is precisely why those should be next.

If you want the underlying biology in more depth, covers the Legionella life cycle and the conditions it needs; for the temperature side, gets into the actual numbers.

Common questions

Can you see or feel biofilm in a pipe or shower hose?

Not without taking the fitting apart, and often not even then with the naked eye. A mature film can be a barely visible slick or a faint sliminess on an internal surface. Clear-looking water tells you nothing about it, which is exactly why control relies on managing conditions — temperature, movement, cleanliness — rather than on how the water looks.

Does a chlorine disinfection get rid of biofilm for good?

No. A disinfection sharply reduces the bacteria present, including some in the film, but it rarely reaches every sheltered pocket, and the film regrows from the survivors. Treat it as a reset within an ongoing control regime, not a permanent cure — the conditions that grew it the first time will grow it again unless they change [3].

If a Legionella sample comes back clear, is the biofilm gone?

No. A negative result describes the water at one outlet at one moment; the biofilm sits on the surfaces the sample never touched. Sampling can support verification or investigation, but HSE is clear that how often you test should follow the system and the risk assessment, not stand in for day-to-day control [5].

Sources

[1] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] 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 [4] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm