Bacteria in drinking water: types, symptoms and limit values

Legionella need oxygen. They live mainly in fresh water and in warm pipes. As water-associated germs, they multiply extremely quickly under the right conditions. They thrive best at 20 to 50 degrees Celsius. Only from around 60 degrees do they die off.

Unlike most pathogens, legionella do not cause infection through drinking. They only become dangerous when inhaled. The sources are water vapour when showering, aerosols in swimming pools and steam baths, air-conditioning systems or cooling towers. In the lungs, they cause two diseases: flu-like Pontiac fever and Legionnaires' disease. The latter is a severe form of pneumonia and is fatal in around 5 to 10% of cases.

For 2018, the Robert Koch Institute (RKI) reported 1.7 cases of legionellosis per 100,000 inhabitants. The real figure is higher. The RKI estimates 18 to 36 cases per 100,000 inhabitants outside hospitals, because by no means every case of pneumonia is tested for legionella. In 2011, the RKI added legionella to the group of the 26 most important pathogens in Germany. The guide to the legionella filter shows which filter is effective on showers and taps.

E. coli and coliform bacteria, colloquially known as coli bacteria, indicate faeces in the water. Escherichia coli normally lives in the intestines of humans and animals. Most strains are harmless. The others have three serious consequences: diarrhoea, fever and kidney failure.

Coli bacteria frequently occur in the Alps and low mountain ranges. There, faecal germs often remain detectable in the water even after the sewage treatment plant. In summer, dry soil exacerbates the problem. Through cracks in the soil, germs from faeces get past “the most important filter layer [the humus]”. The Bayerisches Landesamt für Gesundheit und Lebensmittelsicherheit (Bavarian Health and Food Safety Authority) noted this as early as 2006.

Bottled water does not offer automatic protection either. The same authority also frequently found germs in mineral and table water. Up to 13% of table waters and up to 3% of mineral water bottles were affected, in each case with coliform faecal germs.

Enterococci belong to the lactic acid bacteria and live naturally in the body in small quantities. They also play a role in the ripening of cheese and sausage. However, some species have serious consequences. Like E. coli, they indicate faeces in the water.

They get into groundwater via fertiliser or livestock faeces. Heavy rain washes both into the soil. Breakdowns at the sewage treatment plant or repairs to the pipe network can also wash enterococci into the water.

In people with a weak immune system, enterococci cause infections, especially of the urinary tract, less often of the abdomen and the heart. In women, the fallopian tubes or the bladder sometimes become inflamed. Wound infections and abscesses are also among the consequences. Treatment is sometimes difficult, because some strains resist antibiotics.

Pseudomonas aeruginosa is by far the more dangerous germ in water. It lives almost everywhere and occurs naturally in groundwater. Because it is so undemanding, it resists many antibiotics and disinfectants. It is best known as a multi-resistant hospital germ. Around 10% of all infections in German hospitals are caused by it.

Pseudomonas aeruginosa reaches the cold-water system via the house connection and multiplies there in the biofilm, especially in dead legs or during long periods of stagnation. Once the germ has settled in the pipes, only an elaborate chlorine flush will help.

In humans, the germ causes infections and pus. People with a weakened immune system are at risk of pneumonia, infected wounds and blood poisoning. Healthy people usually only have problems with their skin, nails, hair roots or ear canal. If a laboratory detects Pseudomonas aeruginosa, measures take effect immediately, which can go as far as shutting off the line. In the worst case, the germ infests the entire network.

Multi-resistant germs resist several antibiotics at once. They pass this defence on to the next generation, and the medicine then no longer works. The cause lies in how these drugs are handled: they are used too often, unnecessarily or incorrectly.

The germs enter the environment via wastewater. They survive the sewage treatment plant and flow into rivers with the treated water. Antibiotics used in livestock farming also carry them via slurry onto the fields and from there into streams and lakes. In 2018, research by the broadcaster Norddeutscher Rundfunk (NDR) found that it was “largely unknown” “how heavily bodies of water are contaminated, as there have so far been no systematic checks for such pathogens”. Dr Tim Eckmanns of the Robert Koch-Institut called the findings “really alarming”.

However, multi-resistant germs only reach drinking water from rivers and lakes by a detour. They first have to pass into the groundwater or the bank filtrate. This rarely happens.

Parasites in water are zoonoses, meaning they jump from animals to humans. The two most important are called Cryptosporidium parvum and Giardia lamblia. Their route leads from faeces to the mouth. Both also occur in Germany. The Landesgesundheitsamt Baden-Württemberg (State Health Office) found them in up to 50% of rivers, lakes and karst waters, albeit in small numbers.

In rare cases, parasites trigger epidemics. In Milwaukee in the USA, more than 400,000 people fell ill with severe diarrhoea even though the water had been chlorinated. More than 100 of them died. In people with a weakened immune system, such diarrhoea can be fatal in extreme cases.

Four types of virus make it all the way to the tap: noroviruses, rotaviruses, hepatitis A viruses and echoviruses. They break through the treatment barriers and travel through the pipes into the kitchen. Only a small fraction of all viruses manage this. They enter the cycle via wastewater or via the slurry and manure of sick animals.

Noroviruses are the main focus. They are highly contagious; even a tiny amount is enough. They cause sudden gastroenteritis with projectile vomiting and severe diarrhoea. This is often accompanied by abdominal pain, nausea, headache and exhaustion. There is usually no fever.

A biofilm is a slimy layer of bacteria, fungi and algae. It grows on surfaces and, in the pipes, protects the germs that live in it. There they grow undisturbed and from there pass into the water.

Two things promote biofilm in the home: stagnation and dead legs. Stagnation means that the water stands in the pipe for hours or days, for example after a holiday. A dead leg is a cut-off section of pipe with no flow, for example after a renovation. Both create exactly the calm that germs need to grow. The cause of contamination is therefore often not at the waterworks, but in the last few metres before the tap.

Distillation kills germs by boiling. The water evaporates and the pollutants remain at the bottom. The steam condenses at the top and drips into a container. The process has two drawbacks. The container needs thorough care, and valuable minerals are also lost with the steam.

Reverse osmosis forces water through a very fine membrane, thereby retaining coliform bacteria and other germs. Its pores are even narrower than those of ultrafiltration. That is why it also removes more minerals from the water than an activated carbon filter. It also needs a lot of water, because part of the inflow goes down the drain as concentrate.

Activated carbon only partially retains germs. It depends on its form and on how the filter is built. Loose granules in a jug filter do not stop bacteria. If they are changed infrequently, they even become contaminated with germs themselves, as the article on the risk of germ contamination in jug filters shows. Activated carbon's strength lies in dealing with chlorine, heavy metals and natural residues. The guides What is activated carbon and Activated carbon filters for water explain the basics. The overview of heavy metals in tap water shows what else can be found in tap water.

Ultrafiltration sieves out bacteria purely mechanically. The membrane's pores measure 0.1 micrometres, i.e. 0.001 millimetres. Bacteria are larger and simply cannot pass through. The retention rate is 99.99%. Because activated carbon alone does too little against germs, the two are combined: the carbon binds chlorine and pollutants, and the membrane stops the germs. This is precisely the combination found in the filter cartridge NANO by Alb Filter. It helps in cases of acute germ contamination in the pipes.