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Bacteria and germs in drinking water: What you need to know
In many European regions, water suppliers provide high-quality drinking water and monitor it regularly. Risks usually arise inside the building: in contaminated pipes or when water stagnates, bacteria or other germs can multiply. For operators of drinking water systems this means: in most cases, the supply is not the problem, but rather the installation within the building itself.
In this article, we introduce the most important bacteria found in drinking water and highlight which other germs may also play a role.
Bacteria in drinking water: Water does not have to be completely germ-free
According to the UK Water Supply (Water Quality) Regulations 2016, as amended in 2018, drinking water must not contain any microorganisms or other substances at concentrations that could endanger human health. This means that tap water does not have to be entirely free of bacteria, but there are clear limits that define when action is required. If these limits are exceeded, operators must take appropriate measures to ensure drinking water hygiene, in line with guidance from the Drinking Water Inspectorate (DWI).
Good to know: Standards, indicator parameters and the action level
- Standards (Schedule 1, Water Supply (Water Quality) Regulations): Apply to specific bacteria such as E. coli. For example, E. coli must not be present in 100 ml of drinking water. Even a single detection requires immediate action.
- Indicator parameters (Schedule 2): Used to assess the general condition of the water supply system (e.g. coliform bacteria, colony counts). An exceedance does not pose an immediate health risk but highlights issues that operators should investigate.
- Action level for Legionella (DWI Guidance): There is no fixed regulatory limit in the Regulations themselves, but HSE guidance (L8 / HSG274) sets action levels. The threshold for action starts at ≥ 100 CFU/l. If this level is exceeded, a risk assessment is required and appropriate remedial measures must be taken.
For operators of drinking water systems this means: standards directly protect health, indicator parameters help to evaluate system hygiene, and the action level indicates the risk posed by Legionella.
Especially in healthcare facilities such as hospitals, specific preventive measures and immediate action in the event of detection are required. People with a weakened immune system are at higher risk of contracting diseases from contaminated water.
Does biofilm promote germs in drinking water?
Yes, the formation of biofilm encourages the growth of germs and bacteria, as it provides ideal conditions for pathogens to multiply. Since biofilms are difficult to remove, prevention is particularly important (e.g. through correct system use, hygienic installation, and maintaining adequate temperatures).
Bacteria and germs in drinking water: An overview
A wide range of bacteria and viruses can survive in water, at least for a certain period of time, and drinking contaminated water can cause health problems. Some pathogens can also enter the body through aerosols, the skin, wounds or body openings.
Legionella in drinking water
Legionella are among the best-known bacteria in drinking water.
In particular, the species Legionella pneumophila can cause severe illnesses such as Legionnaires’ disease when contaminated aerosols are inhaled.
Their growth is mainly favoured by temperatures between 25 °C and 45 °C – which is why stagnation in hot water pipes poses a particular risk.
For detailed information, see our article: Legionella in water: prevention and control
Pseudomonads in drinking water
Pseudomonas aeruginosa is a typical waterborne bacterium that requires moisture to grow. It is present in the environment but can also be found in drinking water pipes and biofilms. In these settings it benefits from stagnant water and can multiply rapidly.
For operators of drinking water systems, this bacterium is especially critical because it can cause infections in the lungs, urinary tract or wounds – and it is often resistant to common antibiotics.
For detailed information, see our article: What are pseudomonas: everything you need to know
Coliform bacteria in drinking water
Coliform bacteria such as Escherichia coli are typical indicator organisms. They can only survive in drinking water for a few days – which is why their detection almost always points to a recent faecal contamination.
Even small amounts of water contaminated with certain coliform bacteria can cause diarrhoeal diseases.
For operators of drinking water systems the standard is clear: according to the Water Supply (Water Quality) Regulations, coliform bacteria must not be present in 100 ml of drinking water. Any detection requires immediate action!
For more details, see our article: Coliform Bacteria: E. coli, Enterobacter & more
Stenotrophomonas maltophilia
Stenotrophomonas maltophilia is an environmental bacterium found in soil, on plants and also in water.
In drinking water installations it can colonise biofilms and multiply there.
For healthy people it is usually harmless, but for immunocompromised individuals it can pose a risk, as it may cause infections in the respiratory tract, bloodstream or wounds.
For detailed information, see our article: Stenotrophomonas maltophilia in drinking water: risks, transmission and preventive measures
Klebsiella in drinking water
Klebsiella belong to the family of Enterobacteriaceae and are naturally present in the intestines of humans and animals.
Once they enter a drinking water installation, they can settle in biofilms and persist there.
For healthy people they usually do not cause problems. For immunocompromised individuals, however, they can lead to infections such as pneumonia, urinary tract infections or sepsis.
For more details, see our article: Klebsiella in drinking water: relevant species, risks & protective measures
Enterococci in drinking water
Enterococci are lactic acid bacteria that occur in the intestines of humans and animals, where they support digestion. Due to their fermenting properties they are even deliberately used in some foods, such as cheese or raw sausages.
Enterococci can cause diseases such as urinary tract infections, endocarditis, wound infections or abscesses – especially in people with a weakened immune system or when they enter the body through routes such as catheters, wounds or mucous membranes, for example via contaminated drinking water.
Unlike coliform bacteria, enterococci – which belong to the group of faecal streptococci – can survive outside the intestine for several weeks. Therefore, when contamination is detected, it is difficult to determine the exact time of the incident.
So far, around 25 species of enterococci are known. The most common causes of infections are Enterococcus faecalis and Enterococcus faecium.
Other germs and bacteria in drinking water
There are also other pathogenic germs that can survive in water: these include Salmonella, Enterobacter and Vibrio cholerae. Viruses such as noroviruses and rotaviruses can also cause gastrointestinal illnesses. In addition, there is a risk of infection with hepatitis A and E viruses through drinking water.
Germs in drinking water: What to do? Our recommendations
The key is to ensure that no germs, bacteria or viruses enter the drinking water in the first place.
Prevention with suitable measures is equally important to avoid growth within the installation.
- Let water flow regularly: bacteria multiply particularly easily in stagnant water.
- Clean or replace aerators (tap outlets) regularly – they are typical collection points for germs.
- Have drinking water tested: this allows contamination to be detected at an early stage.
- Use sterile filters: when installed directly on taps or showers, they provide immediate protection against waterborne germs.
Keeping an eye on the most important risks and taking targeted preventive measures is crucial to safeguarding drinking water quality in the long term. For operators of drinking water systems this means: regular monitoring, consistent hygiene and the use of suitable protective measures.