Swedish group develops optical method to rapidly detect contaminated bathing water

17 Dec 2025

Quick test for E. Coli uses laser-based flow cytometry and machine learning.

Testing the water.

Urbanisation and a warmer climate means that more people want to swim in canals, harbors, and at urban beaches. However, this means that they may be swimming close to where treated wastewater and stormwater are discharged – including bacteria, viruses, and other pathogens. A new method developed at Lund University, Sweden, offers both faster and more complete information about whether the water is safe for swimming or not.

A laser-based testing approach created by researchers at Lund University, Sweden Water Research, and Kristianstad University has been successfully tested in Helsingborg, where the test turnaround time has been reduced from several days to just a few hours.

The work is described in a paper in Water Research X.

“Regular monitoring is essential to ensure that the water is safe. E. coli bacteria are so-called indicator bacteria, and are used as a warning sign that microorganisms that can cause illness might be in the water,” said Catherine Paul, associate professor in water resources engineering at Lund University.

Currently, the most common method is to culture E. coli. The results take a few days, which increases the risk of unnecessary exposure to bacteria and potentially also viruses and other pathogens.

The new method instead looks at the community of all bacteria in the water that changes if there is a problem and connects this to the usual measurements for E. coli using machine learning and flow cytometry (a type of liquid laser scanner used to detect cells and other particles) .

Together with two other experts in waterborne bacteria, doctoral student Isabel Erb, from technical microbiology at Lund University and Sweden Water Research, and computer scientist Niklas Gador at Kristianstad University, the team has tested the idea in practice.

Advantages of new method

According to the researchers, the method has several advantages compared with the alternatives on the market:

• Faster – it only takes about 20 minutes to analyse a water sample.
• Cheaper and simpler – requires less work and fewer chemicals than, for example, PCR analyses.
• Automated – the entire process can be handled by a machine, saving both time and staff.
• More sustainable – uses less resources than traditional methods.
• The method uses open-source code.

Flow cytometry is a technique that uses lasers to scan cells and small particles in water samples. Within minutes, it can create a “fingerprint” that identifes all the bacteria in the sample. Software can then interpret the data and provide an 80% reliable answer on whether E. coli is present in the water and in what quantity.

“The method is free to try for anyone who wants to. However, you need access to a flow cytometer, so it’s not for personal use,” said Erb.

Finding source of pollution

Unlike traditional methods, the researchers focus on entire bacterial communities, or microbiomes, which makes it possible to detect changes even if E. coli is absent. “With an overview of the entire microbiome, we may in the future be able to identify the contamination source. The microbiome of bird droppings, for example, looks completely different from that of treated wastewater,” said Erb.

The advantages are many: the analysis takes about 20 minutes, requires fewer chemicals, and can be automated. The machine can measure water samples every 30 minutes. In the future, the goal is for the results to be sent directly to a data system that decides whether a warning should be issued. Expensive and time-consuming tests, such as PCR tests, could then be added for more details and for confirmation.

“The next step is to test the method in more situations, for example in drinking water, and to improve the algorithms for even more reliable predictions,” concluded Prof. Paul.