What the sewer knows: A story of resistance beneath our feet

A Microbial City Below

Imagine the inside of a sewer pipe. It is not just a dirty tunnel. It is a thriving microbial city. While some bacteria come from human waste, many are long-time residents of the sewer itself. They live in flowing water, sediments, and slimy layers called biofilms that coat the pipe walls.

These microbes do more than survive, they interact and exchange genetic material. In particular, they share antibiotic resistance genes through a process called horizontal gene transfer. The sewer environment, rich in nutrients and stable habitats, supports this exchange.

Why Sewers Support Resistance

The review outlines several reasons why sewer systems help antibiotic resistance spread:

• Sewer residence time: Wastewater can remain in pipes for hours, giving bacteria time to grow and transfer genes
• Sediments and metals: Elements like copper and zinc accumulate in slow-moving water and may promote resistance even without antibiotics
• Biofilms: These microbial layers provide a stable setting where bacteria attach and share genetic material

Hospitals as Hotspots

Not all wastewater has the same impact. Hospital effluent contains higher levels of antibiotics and resistant bacteria. When that waste enters the sewer, it can tip the balance. Genes like ermB (linked to azithromycin resistance) and sulfonamide resistance genes are often found in this waste.

Yet, the link between antibiotic concentration and resistance gene abundance is complex. Resistance can spread even when antibiotic levels are low, influenced by biofilms, microbial diversity, and other contaminants. While the overall impact of hospital wastewater varies, in areas with limited dilution and long sewer travel times, it can create local hotspots for resistance. This makes focused monitoring and pretreatment especially important.

‍

Looking for Clues in the Flow

Wastewater is increasingly being used as a tool to monitor antibiotic resistance in communities. But the review warns that much of the genetic exchange may happen within the sewer system — long before the water reaches a treatment plant. Relying only on samples from treatment facilities could miss critical upstream activity.

Factors like temperature, pipe material, water flow, and microbial interactions all influence what gets detected. To capture accurate data, it is essential to understand what is happening throughout the sewer network, not just at the end point.

What Can Be Done

The review highlights two key strategies:

• Pretreating high-risk wastewater, especially from hospitals, before it enters sewer systems
• Disrupting biofilms to limit gene sharing between bacteria

Approaches such as ultraviolet light, peracetic acid, and biological treatments that block bacterial communication show promise. Still, more research is needed, as these methods are not yet fully understood and must balance targeting harmful bacteria while preserving beneficial ones.

Listening to the Sewer

Sewer systems are more than just underground infrastructure, they are living, evolving ecosystems. As this review shows, resistance is quietly taking shape beneath our feet. By better understanding these microbial environments, we can improve surveillance systems, respond more effectively to resistance threats, and build stronger public health protections.

In the fight against antibiotic resistance, even the bacteria hiding in sewers can tell us something important.

For a deeper look at the research behind this story, read the full review article

https://doi.org/10.1016/j.wroa.2025.100378

‍