What is biofilm and why does it matter for legionella?

Biofilm is a thin, slimy layer of micro-organisms that forms on almost any surface which stays wet, including the inside of your pipes, tanks and fittings. It matters for legionella because it shelters the bacteria from heat and disinfectant, feeds them, and gives them a place to hide and regrow. Understanding biofilm explains why almost every other control exists.

What biofilm actually is

Biofilm is not a single organism. It is a mixed community of bacteria, algae, fungi and protozoa that attach to a wet surface and wrap themselves in a sticky, self-produced matrix, sometimes called extracellular polymeric substance or simply slime. That matrix glues the community to the pipe wall and to each other, and it acts as a physical shield. Biofilm can begin to form within hours on any surface that stays wet, and it develops readily throughout hot and cold water systems: on tank walls, inside pipework, around washers and tap seats, and on shower fittings. HSG274 treats biofilm as a central factor in legionella growth, not a rare fault to be surprised by.

Why biofilm is a problem for legionella

Legionella rarely survives for long as free-floating cells in flowing water. It thrives inside biofilm and inside the amoebae and protozoa that graze on it, which act as living incubators. The biofilm does three things that make legionella dangerous.

• It provides nutrients. The matrix traps organic material, so the bacteria have food to multiply.
• It provides protection. The slime slows the penetration of heat and chemicals, so a treatment that clears the open water may leave protected colonies intact.
• It provides a reservoir. Even a small surviving patch can shed bacteria back into the flowing water and re-seed the system.

This is why legionella is better described as a biofilm problem than simply a water problem. The same slime layer is home to other opportunistic organisms too, such as stenotrophomonas maltophilia, which is one reason pseudomonas in water systems and legionella are often discussed together.

Why a one-off disinfection is not the end of the story

Here is the point many pages skip. You can disinfect a system, take a clear sample, and still be re-seeded weeks later. A chemical dose or a heat treatment kills the bacteria it reaches in the free-flowing water, but it struggles to reach organisms buried deep in established biofilm. Survivors regrow, the matrix re-forms, and the surface is re-colonised. A clear test result is a snapshot of one moment, not proof that the system has been made permanently clean.

This is exactly why legionella treatment and chlorination are described as control measures to be repeated and verified, not one-time cures. Disinfection is most effective after loose scale, sediment and slime have been physically removed, so the chemical can actually reach the surface it needs to act on.

Where biofilm builds up

Biofilm builds fastest where two conditions meet: water that sits still, and water that sits warm. Both feed it. Stagnant water in a dead leg, a rarely used outlet or an oversized storage tank gives biofilm time to establish without being flushed away. Warmth accelerates everything, because the growth range for legionella runs from roughly 20°C to 45°C. A length of cold pipe running warm, or a hot pipe that has cooled, is exactly the environment biofilm and legionella prefer.

This is the real reason dead legs and stagnation matter so much. They are not just idle pipe: they are incubators. It is also why the temperature regime is written the way it is. Cold water is kept below 20°C and hot water stored at 60°C, reaching 50°C at the outlet within a minute (55°C in healthcare), precisely to keep the system out of the band where biofilm and legionella flourish.

How control actually works

Because you cannot sterilise a system for good, control is about denying biofilm the conditions it needs and physically removing it where it takes hold. Three levers do most of the work.

1. Keep water moving. Turning water over flushes out nutrients and bacteria before they settle. Regular flushing of low-use outlets is the everyday version of this, and removing redundant pipework takes away the still water altogether.
2. Keep temperatures right. Holding hot water hot and cold water cold keeps the system out of the growth range. Our page on temperature control sets out the figures and how to monitor them.
3. Clean physically. Scale, sediment and slime shield biofilm from any chemical, so cleaning and descaling matter. Cleaning and disinfecting cold water storage tanks and descaling shower heads physically removes the surfaces biofilm grows on.

Chemical treatment supports these steps but does not replace them. A written scheme of control ties the whole approach together, setting out who does what and how often, so control is continuous rather than a one-off event.

Biofilm and the guidance

The Approved Code of Practice L8 and the technical guidance in HSG274 both frame legionella control around keeping water moving, holding the right temperatures and keeping systems clean. That is another way of saying: manage the biofilm. Neither document sets a figure for how much biofilm is acceptable, because the practical answer is to control the conditions continuously rather than chase a number. Your risk assessment should identify where stagnation, warmth and hard-to-clean fittings let biofilm build, and your monitoring should keep checking that those controls are holding.