Bacteria in drinking water - what harms, what not?

Our drinking water is not completely germ-free. This is not required by the Drinking Water Ordinance. What types of bacteria are dangerous in drinking water, and why, and when a risk of infection exists, is explained here.

Types of bacteria

Bacteria can be divided into different groups and strains. Not all bacteria are dangerous to the human body. By far the majority are harmless even in higher concentrations.

Pathogen, that is, disease-causing, only certain bacterial strains act. They can lead to severe infections either through their toxic degradation products or through their metabolic activity in the human body.

Particularly dangerous are enterobacteria. They occur in the human intestine, as well as in faeces. Some dangerous strains, such as Escherichia coli, can cause very serious illnesses throughout the body.

Design of limit values

The limit values ​​in the Drinking Water Ordinance are designed in such a way that the risk of individual bacterial strains is taken into account. Coli bacteria and other enterobacteria strains must not be detectable in drinking water. In technical terms, this group is called coliform germs.

For the other types of bacteria limit values ​​of usually 100 cfu / 100 ml apply. This means that in 100 ml of water there can be no more than 100 colony forming units (CFU) of bacteria. Above this value one speaks of a contamination. The drinking water must be disinfected.

As a rule, the risk of infection is considerably more than 10,000 CFUs - that is more than 100 times the applicable limit value. For patients with a weakened immune system and for children, the risk of infection may also be lower.

Indicator bacteria

Some types of bacteria serve as indicators of water pollution and the presence of other bacteria. In microbiological investigations, therefore, not all germs and substances need to be detected individually.


There is always a risk if an initially low concentration of bacteria begins to proliferate. This can be the case, for example, in the case of microbial contamination or germination of a water filter.

For the dangerous Legionella, there are so-called Legionella circuits in hot water appliances in order to prevent microbial contamination.

Tips & Tricks

Effective water disinfection when traveling or camping or hiking can be done by UV disinfection.

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