Legionella Bacteria - Where do they occur?

What is Legionella?

The word ‘Legionella’ describes a group of bacteria (Legionella spp.), commonly found in water. Not only are they found in rivers and lakes, but also in groundwater and drinking water. Even with the UK’s high standards of water treatment, Legionella bacteria can be present when you turn on your tap or shower.

There are over 40 species of Legionella bacteria, the most common of which causes Legionnaire’s disease. Named after an outbreak at the 1976 American Legion convention in Philadelphia, USA, Legionnaire’s disease is an ongoing concern for facility manager, employers and building owners across the country.

How dangerous is it?

Legionnaire’s disease is a severe form of pneumonia, which can be contracted within 2 to 10 days after exposure. On average, there are about 200 cases of Legionnaire’s disease in the UK each year, with 10 - 20% of those cases being fatal.

Legionella bacteria also causes Pontiac fever. This is a flu-like illness, which can be unpleasant, but is less severe than Legionnaire’s disease.

Where do Legionella bacteria occur?

Poor drinking water quality is caused by, among other things: Dead pipes (pipe sections without a functional tap); a dysfunctional hot water system; germ-laden water flowing back from the waste water to the fresh water area; germ sources in the water tank/drinking water heater; an incorrect water temperature (water that is too cold in the hot water pipes; water that is too hot in the cold water pipes); stagnant water due to shutdowns such as temporary closures; construction phases or when water is not regularly taken from the drinking water pipe.

In the UK, drinking water is assumed to be clean. There are strict guidelines, which the water supply companies follow, to ensure that the water they supply is of an acceptable standard for the general population. But, what happens to that water when it enters public buildings is out of their hands?

The quality of water arriving at the tap or shower outlet can be adversely affected by the water system within a building, and how that water system is managed. Water stagnation can occur when there is little or no movement in sections of pipework for a period of time – such as vacant hotel rooms or school holidays. Water stagnation encourages the growth of biofilms on the internal walls of pipework and also allows the temperature of the water in the pipes to increase which encourages the growth of bacteria and particularly if the temperature of the water rises into the range between 25 – 45°C which is the optimum temperature range for Legionella bacteria growth. If a biofilm has already been created by a different type of bacteria (e.g. Pseudomonas aeruginosa), the Legionella bacteria will utilise it as a safe-haven. This would be the ultimate living condition for the proliferation of Legionella. Therefore, water-tank hygiene, stagnation and temperature are three critical areas which must be addressed, to prevent Legionella bacteria from thriving.

Water outlets such as taps and particularly showers create an ideal environment for legionella bacteria to come in contact with humans. As the shower flows, tiny particles of water (aerosols) mix with the surrounding air and are then inhaled by the user. If the water is contaminated with Legionella bacteria, that contamination is carried into the lungs, and this can cause Legionellosis.

Risk of infection

Everyone is susceptible to infection from Legionella bacteria, but the risk of infection is increased by the type of building, and by the category a person falls into.

According to the HSE, “the risk increases with age, but some people are at higher risk, e.g. people over 45, smokers and heavy drinkers, people suffering from chronic respiratory or kidney disease, diabetes, lung and heart disease or anyone with an impaired immune system”1.

The risk of infection is particularly high in public buildings, such as swimming pools, leisure centres, hospitals and schools, as well as commercial buildings, such as office blocks, hotels and residential complexes where a high ambient temperature may be coupled with protected periods of limited or no use of water outlets.

Understanding the Legionella bacteria

Each Legionella cell has a single polar flagellum. The flagellum is a tail-like protrusion, which spins in water, enabling the cells to move as though ‘swimming’. Unlike Pseudomonas aeruginosa, Legionella bacteria do not form biofilms. However, a biofilm does provide them with an ideal habitat, since it serves to protect them against chlorine, UV light and high temperatures – thus evading three common ways of eradicating bacteria. Biofilms themselves, are difficult to remove, completely. Even thermal or chemical disinfection, and the use of antibiotics can have limited effect on them. In fact, residues of detergents and disinfectants in low concentration can even serve as nutrition.

Not only do biofilms create a safe habitat for Legionella, but they can also cause erratic measurements from microbiological samples. Since parts of the biofilm can break off at irregular intervals so samples taken after each other can give very different results.

The Legionella bacteria have developed a sophisticated strategy for reproducing and spreading, by allowing their natural predator (amoebae) to ‘eat’ (engulf) them, and then exploding out of them once intracellular reproduction is complete. After, the amoebae burst, these new Legionella bacteria are released into the water supply. This is another reason why tests carried out on water can find very different concentrations.

UK regulations governing drinking water

The Health and Safety Executive has published an Approved Code of Practice for Legionella (L8) which provides guidance for Employers and Facility Managers to help them comply with their legal duties. Point-of-use (POU) filters are accepted as an effective barrier to bacteria, when fitted to any water outlet such as a tap or shower. Such POU filters offer reliable protection from waterborne bacteria by preventing it from exiting the water system, thus allowing time for a permanent, safe, engineered solution to be implemented.

UK regulations governing Health and Safety

Under UK law, it is the responsibility of employers to ensure the health, safety and welfare of their employees, to consult with trade union safety representatives, to ensure that non-employees are not exposed to risks, and to ensure that the premises, plant and machinery do not endanger the people using them, so far as is reasonable practicable. Further guidance on these responsibilities may be found on the HSE website (


1 Approved Code of Practice and Guidance on Regulations: Legionnaires’ Disease. Health and Safety Executive