Have you ever left a tap unused for days or even hours? What if you were told that this simple act could turn drinking water into a breeding ground for bacteria and antibiotic-resistant microorganisms? That is precisely what an international team of researchers—led by the Inner Mongolia University (IMU) in China, with participation from the Technical University of Madrid (UPM)—has demonstrated. Their findings show that, beyond tasting unpleasant, stagnant water in pipes can pose serious health risks.
“The aim of the study was to highlight an invisible yet widespread issue that could affect anyone who drinks tap water. We wanted to understand what happens to drinking water when it remains stagnant in pipes for long periods. Specifically, we sought to examine whether small amounts of metals, such as iron and aluminum, influence bacterial growth and whether this, in turn, contributes to the spread of antibiotic-resistant bacteria,” explains Stefanos Giannakis, researcher at the School of Civil Engineering at the UPM and a contributor to the study.
The researchers found that when water remains stagnant in pipes for a long time, bacteria multiply rapidly and form sticky biofilms that adhere to the inside of the pipes
To conduct their research, the team collected tap water samples in Hohhot, a city in northern China with 3.5 million inhabitants, where drinking water is chlorinated and contains residual chlorine. They wanted to analyze what happens when this water remains stagnant over time. After allowing the tap to run for 20 minutes, they collected 2-liter samples, stored them in the dark to simulate stagnation in pipes, and added small glass beads to facilitate bacterial adhesion.
“Over time, we analyzed both the water and the biofilms—those thin bacterial layers that formed on the glass beads—to understand how microorganisms develop in stagnant water. Our goal was to expose invisible risks that could be present in any household and provide evidence to support safer drinking water policies and better plumbing design practices,” explains Professor Ling Feng, lead researcher of the study.
Iron: the most concerning metal
The findings were striking. The researchers found that when water sits stagnant in pipes for extended periods, bacteria multiply rapidly, forming sticky biofilms that adhere to the interior of the pipes. “When small amounts of metals like iron are present, these biofilms become even more dangerous, allowing harmful bacteria like Salmonella enterica and Pseudomonas aeruginosa to thrive,” notes the IMU researcher.
These conditions can also lead to antibiotic resistance, making the bacteria much harder to eliminate. The study highlighted that iron, a metal commonly found in plumbing, plays a key role in strengthening the link between harmful bacteria and resistance genes—posing a serious public health risk.
“This issue isn’t just a scientific concern; it affects the water we drink every day. If you’ve ever turned on a tap and noticed a slight discoloration or metallic taste, it could be a sign that small traces of metal are interacting with bacteria in your pipes,” adds the UPM researcher.
Published in the international journal Clean Water, a Nature partner journal, the study shows that even when water appears clean, it can hide invisible dangers, increasing the risk of infections from antibiotic-resistant bacteria.
According to the researchers, this issue may be particularly worrying in urban homes, vacation houses, schools, hospitals, and other places where water can sit unused for long periods. Their goal is not to create alarm, but to encourage stricter water quality regulations, promote safer plumbing materials, and raise public awareness about the hidden dangers in drinking water.
