Microscopic view of biofilms on a water filter membrane.

Biofilm Alert: Are Water Filters Breeding Superbugs?

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Jordan Keane in Science & Nature December 2025 • 4 min read.

"Research reveals how ultrafiltration membranes in water treatment can become hotspots for bacteria like Salmonella, even surviving cleaning."


Clean water is a global concern, pushing the use of wastewater recycling and seawater desalination. Ultrafiltration (UF) membranes are essential in these processes, acting as gatekeepers to remove contaminants before water is safe to drink. They're favored for their ability to reduce organic matter and prevent biofouling in subsequent treatment stages like reverse osmosis (RO).

But here's the catch: UF membranes themselves aren't immune to biofouling. Bacteria can stick to the membrane surface, forming a biofilm – a slimy layer of microorganisms and their secretions. This not only reduces the membrane's efficiency but also raises a disturbing question: Could these biofilms harbor harmful bacteria, potentially compromising water safety?

A recent study published in Scientific Reports investigated this very issue, exploring the fate of indicator and pathogenic bacteria, such as Salmonella and E. coli, on UF membranes used to treat domestic wastewater. The results reveal how these bacteria can rapidly colonize membrane surfaces and even withstand standard cleaning procedures, with implications for water treatment and public health.

Biofilm City: A Haven for Bacteria

Microscopic view of biofilms on a water filter membrane.

The study, conducted under field conditions, found that various types of bacteria, including heterotrophic bacteria, fecal coliforms, E. coli, and Salmonella, rapidly increased on the membrane surface. The numbers kept climbing throughout the experiment, eventually reaching significant levels.

Researchers observed similar trends under controlled lab conditions, confirming that the increase in bacterial numbers depended on a continuous supply of fresh wastewater. Quantitative real-time PCR (qrt-PCR) further validated the presence of E. coli cells, even revealing higher numbers compared to standard culture-based methods. This suggests that some bacteria may be present but not actively growing, yet still detectable.

Here's a breakdown of what the researchers discovered:
  • Bacterial numbers on the membrane surface increased rapidly and continuously.
  • The increase depended on the availability of fresh wastewater.
  • Quantitative real-time PCR confirmed the presence of E. coli, even in a non-culturable state.
The study also found a link between the number of attached bacteria and other factors. As the amount of DNA and protein on the membrane increased (indicating biofilm growth), the membrane's ability to filter water (flux) decreased. This highlights how biofilm formation directly impacts membrane performance.

Cleaning Isn't Always Enough: What This Means for Water Safety

The researchers also tested the effectiveness of sodium hypochlorite, a common disinfectant, in removing attached bacteria. While the treatment did reduce bacterial numbers, the effect was temporary. Bacteria quickly rebounded, suggesting that they were able to recover and recolonize the membrane surface.

These findings raise concerns about the long-term effectiveness of current cleaning methods and the potential for UF membranes to act as reservoirs for harmful bacteria. It emphasizes the importance of continuous monitoring and optimization of cleaning strategies to ensure water safety.

This research underscores the need for innovative approaches to prevent biofilm formation and control bacterial growth on UF membranes. Further research is needed to develop more effective cleaning methods and explore alternative membrane materials that are less susceptible to biofouling. Ultimately, ensuring the safety of our water supply depends on a deeper understanding of these microbial ecosystems and proactive measures to minimize their impact.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1038/s41598-018-36406-z, Alternate LINK

Title: Fate Of Bacterial Indicators And Salmonella In Biofilm Developed On Ultrafiltration Membranes Treating Secondary Effluents Of Domestic Wastewater

Subject: Multidisciplinary

Journal: Scientific Reports

Publisher: Springer Science and Business Media LLC

Authors: Jeries Jadoun, Raghda Mreny, Ons Saad, Hassan Azaizeh

Published: 2018-12-01

Everything You Need To Know

1

Can water filters themselves become contaminated with bacteria?

Ultrafiltration membranes, while designed to remove contaminants, can unfortunately become breeding grounds for bacteria, leading to biofouling. This occurs when bacteria stick to the membrane surface, forming a biofilm that reduces the membrane's efficiency and can potentially harbor harmful bacteria like Salmonella and E. coli, affecting water safety.

2

Does cleaning water filters effectively remove all harmful bacteria?

The research indicates that even after cleaning with sodium hypochlorite, a common disinfectant, bacteria can quickly rebound and recolonize the ultrafiltration membrane surface. This suggests that the bacteria are able to recover and re-establish themselves within the biofilm, presenting challenges for maintaining water safety.

3

What is 'Quantitative real-time PCR' and why is it important in detecting bacteria on water filters?

Quantitative real-time PCR (qrt-PCR) is a molecular technique that can detect the presence of E. coli cells, even when they are not actively growing and cannot be detected by standard culture-based methods. This is significant because it reveals that even non-culturable bacteria can persist within the biofilm on ultrafiltration membranes, potentially posing a risk under the right conditions.

4

How does the presence of bacteria on water filters affect their performance?

The study found a direct relationship between the growth of biofilms on ultrafiltration membranes and a decrease in the membrane's ability to filter water, known as flux. As the amount of DNA and protein on the membrane increased, indicating biofilm growth, the membrane's performance declined. This emphasizes the importance of preventing and controlling biofilm formation to maintain the efficiency of water treatment processes.

5

What are the broader implications of these findings for water treatment and public health?

Ultrafiltration membranes play a crucial role in removing contaminants from water, but understanding their vulnerability to biofouling is key. While effective in reducing organic matter and preventing biofouling in reverse osmosis, the fact that biofilms can thrive and harbor pathogens like Salmonella and E. coli highlights the need for more robust cleaning and monitoring strategies. Further research should explore innovative methods to prevent biofilm formation and ensure the long-term safety and efficiency of water treatment processes.

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