Bacteria in tap water: should we be worried?

Among the micro-organisms that can contaminate our tap water, in this article we have decided to focus more specifically on the bacteria. Although the quality of drinking water in France is generally good, there are still a number of shortcomings. occasional episodes of contamination are a reminder that vigilance is essential. But what are these bacteria, how do they get into our glass of water, what are the health risks and, above all, what can we do to avoid them?

What bacteria can be found in tap water and what are their effects on health?

Let's start with the main culprits identified:

Escherichia coli (E. coli) Salmonella: a bacterium naturally found in the intestines of animals and humans, so it can be found in faeces. Its presence in water indicates recent, direct contamination. Health effects : Its presence can cause gastrointestinal problems.
Legionella : Bacteria in this category (legionella) like warm environments and can proliferate in poorly maintained domestic hot water systems. Health effects : They can cause legionellosis, a serious lung infection.
Pseudomonas aeruginosa : It can appear in poorly disinfected or stagnant water systems. Effects on health A type of opportunistic bacterium frequently implicated in nosocomial infections, particularly in immunocompromised or hospitalised patients. It can cause skin, urinary and respiratory infections.
Enterococci : Other indicators of faecal contamination, their detection signals possible water pollution. Health effects : Harmless in healthy people. However, they can become pathogenic under certain conditions, particularly in immunocompromised or hospitalised individuals.

While it is important to note that the presence of these bacteria in tap water is rare, if they are present, their dangerousness will of course depend on how much water they contain. concentration in the water. In any case, their presence is a sign to be taken seriously.

What are the maximum permitted levels of bacteria in tap water?

In France, water treatment services put in place rigorous processes to eliminate bacteria, guaranteeing drinking water that complies with health standards. This treatment begins with a mechanical filtrationto retain particles and certain micro-organisms. Secondly, advanced techniques such as chlorinationThe use of chlorine, ozonation or ultraviolet light neutralises bacteria by destroying their DNA or cell membranes.

And we're not joking about it! In fact, the microbiological quality of drinking water is strictly regulated and must meet the following thresholds (example for the main bacteria already mentioned):

Escherichia coli (E. coli) : total absence in 250 mL of water.
Legionella : The concentration of legionella must not exceed 1,000 colony-forming units per litre (CFU/L) at points of use of water accessible to the public.
Pseudomonas aeruginosa : absence required in 250 ml of water
Enterococci : total absence in 250 mL of water.

These criteria ensure that tap water is free from recent faecal contamination, for example, thereby guaranteeing its potability.

Is chlorine effective? Yes, but...

Chlorination, which is widely used in France, is an effective method for disinfect the water and eliminate bacteria. However, there is a downside to this process: it can alter the taste and smell of the waterThis makes tap water sometimes unpleasant for consumers.

These inconveniences, although safe for your healthAs a result, many households are, at best, looking for ways to improve the perceived quality of their water, and at worst, are buying water treatment plants. plastic water bottles. Remember that bottled water costs on average 100 times more expensive than tap water, is a real ecological disaster and also contains harmful substances. microplastics and even nanoplastics. The cup is full!

There are solutions for eliminating chlorine

This is where schemes like the LAVIE water purifier find their use. Using advanced technologies, this type of system makes it possible to neutralise chlorine odours and taste while retaining the minerals essential for optimal hydration. This offers an environmentally-friendly, simple and effective alternative for those who want top-quality water at home.

But let's get back to the main subject: how can bacteria still be found if they have been virtually all eliminated by water treatment services?

How do bacteria get into tap water?

If these little creatures can slip through the (small) net, it may be due to a number of factors:

Ageing distribution networks : Leaks in old or poorly maintained pipes can introduce not only E. coli, but also other bacteria such as Pseudomonas aeruginosa and total coliforms.
Work or bad weather : Disruptions to the water system, such as repairs or flooding, can open the door to contamination by a variety of pathogenic bacteria.
Poorly maintained reservoirs or private facilities : Bacteria such as Legionella, which like warm environments, thrive in stagnant or inadequately cleaned water systems.
Problems with the source or processing : Although rare, errors or faults in the water treatment process can let bacteria through.
Specific survival conditions : Some bacteria, such as Pseudomonas, are particularly resistant to disinfectants such as chlorine, enabling them to survive even in well-treated systems.

How can you tell if your tap water is contaminated with bacteria?

Here are a few simple steps to check the bacteriological quality of your water:

Consult the water quality reports Published by your local authority, they provide an overview of the regular checks carried out.
(To find out more, read our article Tap water quality in France: we tell you everything you need to know)
Use home bacteriological tests: available on the market, these kits allow rapid, albeit approximate, verification.
Have a sample analysed in a laboratory: This method is the most reliable for detecting the presence of micro-organisms.

Finally, clues such as a suspicious odour or recurring digestive problems can be warning signs.

How can bacteria be eliminated from tap water?

First of all, if you are faced with a proven bacterial contamination, the immediate measures are :

Boil the water for at least 1 minute before drinking.
Clean pipes, taps and water heaters.

For the long term, you can turn to purifiers and home water filters available on the market. Here are a few devices that combat bacteria in tap water:

UV-A filters : These devices limit bacterial division (reproduction by division) by modifying their DNA, but do not alter the mineral composition of the water.

tap water filtered by the LAVIE water purifier — LAVIE UV-A water purifier

Ceramic filters : With their ultra-fine pores, they retain certain micro-organisms, but require regular maintenance.
Domestic chlorination : Although very effective, this method will, as we discussed earlier, alter the taste of the water.
Osmosis (reverse osmosis) : use a semi-permeable membrane to eliminate bacteria, viruses and other contaminants. There are major drawbacks to this system: high water consumption, costly installation, energy and environmental costs. What's more, bacteria can colonise the reservoir or the filters at the outlet of the reverse osmosis membrane.
Activated carbon filters : eliminates bacteria, odours and the taste of chlorine, but is less effective against persistent bacteria. To find out more, click here: Activated carbon for filtering tap water: benefits, limitations and effective alternatives
Combined systems : combine several technologies (such as activated carbon filters and UV-A filters) for maximum purification, but require a slightly higher initial investment.
Water distillers : heat the water to kill bacteria before condensing the steam. Maximum purification means maximum energy consumption!

Read also: The most common water filtration methods: advantages and disadvantages

Finally, what about filter jugsoften presented as a simple and accessible solution?

Filter carafes under the microscope

Although filter carafes are attractive for their promise of healthier water at a lower cost, they also have the following disadvantages significant limitationsparticularly in terms of bacteriology. According to theUFC Que Choisir and ANSES, these devices can become genuine "bacteria nests if they are not used correctly. So, far from improving water quality, improper use can compromise it even further + link

Furthermore, the overall effectiveness of filter carafes is often judged to be relative. While they can reduce certain impurities, they do not guarantee total protection against the various contaminants. This finding calls for a prudent use and more reliable solutions for treating water at home.

Read more: Filter carafes: a good idea?

Other micro-organisms in tap water

In addition to bacteria, other micro-organisms can also be found in tap water. These organisms include :

Viruses : such as noroviruses, which are known to cause gastro-enteritis. Although their presence in drinking water is rare, occasional contamination can occur, particularly after flooding.
Parasites: such as Giardia and Cryptosporidium, which are resistant to chlorine and can cause intestinal infections.

Here too, these micro-organisms are often introduced into water systems as a result of environmental contamination or inadequate treatment of raw water. Their detection requires specific analyses, and their elimination relies on advanced techniques such asultrafiltration or theozonation.

Summary table of micro-organisms in water that can contaminate drinking water

Micro-organism	Origin / Risk factor	Signs of presence	Effects on health
Escherichia coli (E. coli) (bacterium)	Faecal contamination (human or animal)	Unusual odour, digestive problems	Gastroenteritis (diarrhoea, nausea), abdominal cramps
Legionella (bacteria)	Stagnant hot water, poorly maintained water heater	No particular visual signs (specific analysis required)	Legionellosis (a potentially serious respiratory infection)
Pseudomonas aeruginosa (bacterium)	Poorly disinfected or stagnant water systems	Unpleasant smell or taste	Skin, urinary tract and respiratory infections
Total coliforms (bacteria)	Presence of organic matter, old networks	Changes in the taste or smell of the water (cloudy water)	Generally harmless (but possible indicators of contamination)
Salmonella (bacteria)	Water contaminated with faecal matter	Rare in France, gastrointestinal symptoms	Typhoid fever, severe gastroenteritis
Enterococci (bacteria)	Sewage infiltration	No direct demonstration (laboratory analysis)	Intestinal and urinary tract infections
Giardia (parasite)	Raw water insufficiently treated	Digestive problems (diarrhoea, bloating)	Giardiasis (intestinal infection)
Cryptosporidium (parasite)	Contaminated water, high resistance to chlorine	Persistent digestive problems	Severe diarrhoea, increased risk for immunocompromised people
Norovirus (virus)	Contamination of faecal or environmental origin	Localised epidemics, acute digestive symptoms	Acute gastroenteritis (vomiting, diarrhoea)

In conclusion, although the presence of bacteria in tap water is controlled and rare, it cannot be ruled out. By adopting preventive measures and monitoring the quality of your water, you can limit the risks. So stay vigilant and enjoy your tap water in complete safety!

Article written on 20/01/2025 by Pascal Nuti - CIO Solable - Passionate about energy, I'm constantly on the lookout for new targets for improvement, exploring cross-disciplinary paths using novel methods.