Legionella are gram-negative, aerobic bacteria that occur naturally in small numbers in non-salt surface waters and soil. Under certain conditions, Legionella can multiply in water-carrying pipes and fixtures. The optimal temperatures for this are between 25°C and 45°C.
Legionella are classified as potentially pathogenic to humans; inhalation or microaspiration of water droplets containing Legionella can cause Legionnaires’ disease, a form of pneumonia, or Pontiac fever in humans.
According to the currently valid German drinking water regulation (TrinkwV), business owners and other operators of a water supply system are required to have the drinking water tested for Legionella as part of a systemic examination in buildings if the drinking water is dispensed as part of a commercial or public activity, if a large drinking water heating system is present, and if the water supply system contains showers or other facilities that result in the atomization of the drinking water.
A large drinking water heating system is present if the drinking water heater has a storage volume of more than 400 liters or if there is a pipeline with a capacity of more than 3 liters between the outlet of the drinking water heater and the point of withdrawal.
Note: Obligations for Legionella testing may also arise from other legal areas besides the German drinking water regulation.
If the drinking water is supplied as part of a public activity, the systemic tests for legionella must be carried out annually
The health department can extend the interval up to three years under certain conditions.
If the drinking water is supplied as part of a commercial activity, but not a public activity, the systemic examinations for legionella must be carried out at least every three years.
If one of the above-mentioned systems is put into operation for the first time, a legionella examination must be carried out within 3 – 12 months after commissioning in accordance with § 14b para. 6 TrinkwV .
According to the currently valid Drinking Water Ordinance , samples must be taken at several representative and suitable sampling points as part of a systemic legionella examination in accordance with the recommendation of the Federal Environment Agency dated December 18, 2018 . Samples are to be taken at the drinking water inlet and outlet of the drinking water heater as well as in the periphery to detect each riser.
If the technical action value of 100 CFU per 100 milliliters is exceeded in the course of a systemic examination for Legionella, the laboratory is obligated according to the currently valid Drinking Water Ordinance  to inform the responsible operator as well as the responsible health authority.
Drinking water samples must be taken as part of an analysis for legionella in accordance with the Drinking Water Ordinance  by testing laboratories accredited for this purpose and approved in accordance with § 15 Para.4 Drinking Water Ordinance .
 Trinkwasserverordnung in der Fassung der Bekanntmachung vom 10. März 2016 (BGBl. I S. 459), die zuletzt durch Artikel 1 der Verordnung vom 22. September 2021 (BGBl. I S. 4343) geändert worden ist.
 Systemische Untersuchungen von Trinkwasser-Installationen auf Legionellen nach Trinkwasserverordnung – Probenahme, Untersuchungsgang und Angabe des Ergebnisses. Empfehlung des Umweltbundesamtes vom 18. Dezember 2018
 Trinkwassererwärmungs- und Trinkwasserleitungsanlagen – Technische Maßnahmen zur Verminderung des Legionellenwachstums – Planung, Errichtung, Betrieb und Sanierung von Trinkwasser-Installationen, DVGW-Arbeitsblatt W 551, Berlin: Beuth-Verlag, April 2004