Bacteria in water: discover the most common pathogens


Bacteria in water: what germs contaminate our water? Tap water in many European regions in characterised by its high quality and is subject to regular testing. Nonetheless, bacteria, viruses or germs can still be found in it. In most cases, this has nothing to do with the quality of the water fed into the system by the suppliers, but with contaminated pipes. There are many types of bacteria and germs that can be found in water, but, thankfully, there are just as many methods to prevent them from multiplying.

Water bacteria: water does not have to be germ-free

Even though there is no universal limit for the amount of CFU (colony-forming units) per milliliter of water, the Water Supply (Water Quality) Regulations from 2016 state that there should be zero concentration of Enterococci or E. coli bacteria per 100ml of water. However, this does not mean that our tap water must be completely germ-free. A distinction should be made between healthy individuals and those with weakened immune systems, particularly in healthcare facilities. Special measures are required in such settings, as people with weakened immune systems are at higher risk of illness from contaminated water. Consequently, there are strict regulations to prevent infections in hospitals.

Does biofilm favour microbes in water?

The formation of biofilm favours the proliferation of germs and bacteria since it provides ideal conditions for pathogens to multiply. We distinguish between pathogenic microbes that are able to survive in water and germs that live and multiply in water as single cells or in biofilms.

Biofilms should therefore be regularly removed from pipes and tubes by experts.

Microbes and bacteria in water: an overview

Many bacteria and viruses can survive in water, at least for a certain amount of time. Drinking contaminated water can cause infections in humans. Some pathogens can also be absorbed through the skin, wounds or body orifices.

Legionella in tap water

Legionella is probably the best-known bacteria that lives in water, and it is considered a human pathogen (i.e.: potentially dangerous to humans). More than 48 species of legionella have been identified. Out of all of these, the most dangerous for humans is Legionella pneumophila, which is responsible for about 90% of all infections, especially Legionnaires‘ disease.


Legionellae multiply intracellularly in amoeba and other protozoa that form biofilms in water pipes. This process depends heavily on the water temperature. The ideal conditions for the bacteria are between 25 and 55°C. This means that older, rarely used hot water pipes pose a higher risk of contamination.

It is important to distinguish between two phases in the life cycle of the legionella bacteria:

  • In the replication phase, the bacteria are motionless and have a low toxicity.
  • In the second phase, the bacteria are thicker and shorter and have formed flagellae. In this phase, they are particularly pathogenic and therefore dangerous to humans. Infection occurs through inhalation of aerosols containing legionella and the subsequent multiplication of the bacteria in the lungs.

Pseudomonas in tap water

Pseudomonas such as the Pseudomonas aeruginosa are waterborne bacteria found in damp environments; in nature mainly in soils and riverbanks. They can also live in garden hoses or water pipes.

The rod-shaped, aerobic bacteria of the genus Pseudomonas aeruginosa are responsible for many human infections and are nowadays considered the most common germs in hospitals. They can be dangerous for humans, since they cause many different diseases such as pneumonia, urinary tract infections, acute inflammatory bowel disease, meningitis and middle-ear infections. Furthermore, they are also resistant to many common antibiotics. Stagnant water provides the ideal living conditions for pseudomonas. Biofilms can easily form here, providing a good source of nutrients, protection from microorganisms and promoting the reproduction of the bacteria.

Coliform bacteria in water

Coliform bacteria such as E. coli (Escherichia coli) can sometimes be found in tap water. However, unlike waterborne microbes like legionella and pseudomonas, this pathogen can only survive in water for a few days. If E. coli is detected in water, this indicates an acute or at least recent contamination with faeces. Drinking large amounts of water contaminated with coliform bacteria can cause diarrhoea.

The maximum permitted value of E. coli in drinking water is 0.

Enterococci in tap water

Enterococci are lactic acid bacteria found in human and animal intestines, where they help with digestion. They are sometimes added to food such as cheese or sausage due to their fermenting properties. Unlike coliform bacteria, enterococci, which belong to the subgroup of faecal streptococci, can survive outside of the intestine for up to several weeks. This means that, if contamination is detected, it is hard to establish when exactly it took place. There are around 25 species of enterococci known to date. The most common pathogens among them are Enterococcus faecalis and Enterococcus faecium. Typical diseases associated with enterococci are urinary tract infections, heart inflammation (endocarditis), wound infections or abdominal abscesses.

💡 There are other pathogens that survive in water. These include salmonella, enterobacter and cholera bacteria, as well as viruses such as noroviruses and rotaviruses, which can cause gastrointestinal illnesses in humans. All of these can enter the body through tap water. There is also a chance of becoming infected with hepatitis A and E by drinking tap water.

Microbes in water: what to do? Here are our tips

  • Let water run. Bacteria thrive in stagnant water, so it is important to ensure that water does not stand in the pipes for too long.
  • Clean or replace aerators regularly. The strainer on the faucet is called an aerator and is a surface on which germs can form.
  • Test the water. Bacteria can go unnoticed for a long time. If you want to check the quality of your drinking water quickly and reliably, the best solution is to have it tested.
  • Use a filter. Sterile water filters can be fitted directly to the tap or shower and offer an immediate, safe protection against water bacteria.