Legionella are basically found wherever there is fresh water – so they are an integral part of our drinking water in small numbers. Nevertheless, legionella can pose a threat to human health, as they can cause severe pneumonia, among other things, if they become infected.

If the external conditions are optimal for Legionella growth, they can multiply rapidly. But what exactly are legionella? How are they transmitted and how fast do pathogens actually multiply in drinking water? All this and how to protect yourself from the dangers of legionella, you will learn in this article.

What are legionella?

Legionella are rod-shaped bacteria of the Legionellaceae family that occur naturally in water bodies such as surface waters, lakes and also groundwater. Mainly they are found in fresh water or also moist soil. Salt water, on the other hand, does not provide an optimal living space for legionella, so they do not occur there.

Legionella prefer water temperatures between 25 and 45 degrees. At low temperatures below 20 degrees, however, the pathogens multiply only gradually or not at all. Legionella are usually killed at temperatures above 60 degrees.

So far, about 48 Legionella species and among them about 70 serogroups are known. The genus Legionella pneumophila poses a particular threat to humans. Among other things, the pathogen can cause serious pneumonia, such as Legionnaires’ disease, or Pontiac fever, which is somewhat milder and causes flu-like symptoms.

If the growth conditions for legionella are optimal, they can multiply rapidly and then eventually become harmful to human health.

How they are transferred

Legionella is usually transmitted via contaminated water droplets. They can become airborne and be inhaled via atomized or nebulized water – such as the water vapor produced during showering. Thus, the legionella enter the lungs directly, resulting in legionella infection.

This legionellosis often manifests itself in the form of Pontiac fever or also as so-called Legionnaires’ disease. The latter is a particularly severe form of pneumonia that must be treated immediately with antibiotics. Otherwise, long-term damage from the disease cannot be ruled out.

Because Legionella spreads through airborne droplets containing the pathogen, showers, hot tubs, faucets, air conditioners, and humidifiers are considered potential sources of infection. Drinking water contaminated with legionella, on the other hand, should not usually pose a risk of infection, according to experts – as long as the pathogens do not enter the lungs via the trachea. Also, human-to-human infection should not be possible.

Learn more about the symptoms of Legionella infection.

Legionella grow quickly

Legionella are considered heat-loving bacteria. For this reason, they feel most comfortable at temperatures between 25 degrees and 45 degrees.

Studies have shown that Legionella proliferate at certain temperatures as follows:

• up to 20 degrees → alive, no growth
• 20-25 degrees → slow growth
• 25-30 degrees → fast growth
• 30-40 degrees → optimal growth
• 40 to 45 degrees → fast growth
• 45 to 50 degrees → slow growth
• 50 to 60 degrees → slow dieback
• from 60 degrees → fast dieback
• From 70 degrees → safe die-off

Particularly ideal conditions for Legionella growth apply in water pipes of buildings. Due to a lack of ventilation, a so-called biofilm often develops there. This represents the optimal breeding ground for germs and microorganisms of all kinds – including Legionella. Combined with the optimal temperature or even water stagnation, Legionella can multiply to over 16 million cells in about eight hours – assuming the necessary nutrients are present.

How are they formed?

Basically, legionella does not originate in water, but it is an integral part of drinking water and other bodies of water. In small numbers, they are largely harmless. Legionella usually only cause illness when the pathogens multiply too quickly and reach concentrations of 100 CFU per 100 ml of water.

Water temperature plays a central role in the growth of legionella. In addition, the nutrients of biofilm , as well as stagnant water in poorly maintained or infrequently used water pipes, can promote the proliferation of Legionella. The amoebae and other protozoa contained in the biofilm provide the ideal breeding ground for Legionella, as they reproduce intracellularly in protozoa.

How to protect yourself

Legionella prefer to multiply under the following conditions:

• warm water (25 to 45 degrees)
• Standing water (water stagnation)
• Biofilm in the water pipes (optimal breeding ground)

However, it is certainly possible to take some measures to contain the growth of legionella so that the water does not become contaminated with legionella, which is a health hazard.

The following measures represent appropriate legionella protection:

1. Observe hot water temperature: From a temperature of about 50 to 60 degrees the legionella die slowly. For this reason, the hot water temperature in the water tank and pipes should ideally never drop below 55 degrees. In the best case, the drinking water installation is equipped with an automatic legionella circuit so that the water heats up to about 70 degrees once a week to kill the pathogens. This process is also known as thermal disinfection, among other things.
2. Avoid water stagnation: Stagnant water can also lead to rapid Legionella growth. To avoid excessive concentration due to standing water, all water lines should be used regularly. For example, the barely used faucet in the guest toilet should be turned on for a few minutes at least every three days so that the water is completely replaced once.
3. Removing biofilm: The biofilm in water pipes is the optimal breeding ground for Legionella bacteria, which multiply intracellularly. Therefore, the formation of any deposits in the water pipes should be avoided as far as possible from the outset. If pipes are shut down, it is essential to ensure that there is no more water in them. In addition, any biofilm already present should be removed immediately.

FAQ

When do legionella form in water pipes?

Legionella are basically always present in the water pipes. At temperatures between 25 and 45 degrees, however, the growth of the bacteria is stimulated, so that a higher concentration then occurs.

When do legionella form in the boiler?

Legionella also multiply more in the boiler at comfortable water temperatures of 25 to 45 degrees.

When do legionella develop in the shower?

In the shower, legionella usually develops in increased numbers when the shower has not been operated for a long time. Deposits in the shower head or supply lines, as well as the optimal water temperature, further promote growth.

How long should you run water?

In case of non-infestation or low values: A water temperature of 70 degrees is required for thermal disinfection. The hot water is flushed through the pipes for about three minutes so that the legionella die off.

In case of legionella infestation or suspicion: Immediately contact professionals who will perform disinfection.

Conclusion

Legionella are part of the natural aquaflora, but high concentrations can cause serious illness.

In order to prevent rapid growth of the pathogens, the appropriate measures for legionella protection should be followed. However, with a legionella circuit or a legionella filter in the drinking water system, there is no need to worry and you can enjoy both the hot shower and the drinking water without hesitation.