What Causes Legionella to Grow?

Legionella multiplies when water sits between 20 and 45°C, stagnates without regular flow, and finds a foothold in biofilm, scale, sediment and organic nutrients inside pipework. These conditions cause growth, not the bacteria's mere presence. Remove any one and multiplication slows sharply.

What causes legionella to grow? The five conditions

Legionella bacteria live naturally in rivers, lakes and soil at low, harmless numbers. A problem only develops when a man-made water system recreates the conditions the bacteria need to breed. There are five, and they usually act together.

• Warm water in the 20 to 45°C range, the band in which the bacteria actively multiply.
• Stagnation, where water stands long enough to lose disinfectant and let colonies settle.
• Biofilm, the slime layer on pipe and tank surfaces that shelters legionella and the amoebae it lives inside.
• Scale and sediment, which give the bacteria surface area to colonise and shelter from heat and biocide.
• Nutrients such as rust, organic matter and corrosion products that feed the colony.

For the organism itself, see what legionella is, and legionella in water for where it comes from. This page focuses on the conditions that turn a normal, low-level presence into dangerous multiplication.

Temperature is the primary cause

Temperature is the single biggest driver of legionella growth, and HSE guidance sets the boundaries clearly. Below 20°C the bacteria are effectively dormant and do not multiply. Between 20 and 45°C they multiply, with the fastest growth around 35 to 37°C, close to body temperature. At 60°C, stored hot water kills legionella within minutes.

This is why the temperature regime in ACOP L8 and HSG274 exists: keep cold water below 20°C, store hot water at 60°C, and deliver it to outlets at 50°C, or 55°C in healthcare settings. Every lukewarm tank, long uninsulated pipe run, or cold-water cistern warmed by a nearby boiler pushes water into the growth window and becomes a cause in its own right. See legionella temperature control for how the regime is applied and monitored.

HSE guidance in HSG274 Part 2 and the ACOP L8 approved code of practice

Stagnation: the driver most systems get wrong

Stagnation is the second major cause, and the one most systems get wrong. Moving water carries residual disinfectant and does not give colonies time to establish. Standing water does the opposite: disinfectant decays, temperature drifts towards ambient, and biofilm builds undisturbed.

The usual culprits are dead legs and redundant pipework, infrequently used outlets such as a spare bathroom or an end-of-corridor tap, oversized storage tanks that hold more water than the building turns over, and whole buildings left empty over a void period or holiday. Weekly flushing of little-used outlets and removing pipework that no longer serves a purpose both target this cause directly.

Biofilm, scale, sediment and nutrients: what feeds the colony

Warmth and stagnation set the stage. Biofilm, scale, sediment and nutrients let the colony dig in. Biofilm is a slime layer that forms on almost every wetted surface. It shelters legionella from heat and biocide and houses the free-living amoebae that legionella infects and breeds inside, which is why it matters far more than its thin appearance suggests. See biofilm in water systems for detail.

Scale, from hard water, and sediment, from rust and debris, add surface area and pockets where disinfectant cannot reach and where temperatures stay lukewarm. Organic matter, corrosion products and rust act as nutrients that feed both legionella and the wider microbial community it depends on. Poorly maintained storage tanks, dirty showerheads and corroding pipework combine all of these at once, which is why they appear on almost every list of legionella risks.

Presence is not the cause: the distinction that matters

The most common misconception is that finding legionella, or the bacteria simply being present, is the cause of the problem. It is not. Legionella is present at low levels in most water systems, and a single positive sample does not mean the system caused disease. The cause is the combination of conditions above that lets numbers climb from harmless to hazardous.

This is why sampling uses action levels rather than a simple present-or-absent test. Under HSG274, a hot or cold water result between 100 and 1,000 CFU per litre calls for review and resampling, and a result above 1,000 CFU per litre calls for immediate action. The figure measures how far the growth conditions have been allowed to run, not merely whether the bacteria exist. See legionella sampling and action levels for what the numbers mean and what to do if legionella is detected.

How each control removes one cause of growth

The value of naming the causes is that each maps to a specific control. Temperature control removes the warm-water window. Flushing and removing dead legs remove stagnation. Cleaning and disinfection, including chlorination, strip out biofilm, scale and sediment. Material choice and corrosion control cut off nutrients. No single measure is enough on its own, because the causes act together, which is why the law expects them to be managed as a set under a written scheme.

A written scheme of control records which control addresses which risk at each part of the system, and a legionella risk assessment is what identifies where each condition is present in the first place. Neither a control nor a clear sample proves a system is safe. They show that the known causes of growth are being identified, recorded and kept in check.