# Microplastics as active modulators of Escherichia coli biofilm characteristics and their implications on the development of antimicrobial resistance

## TL;DR

This study shows that microplastics help E. coli form thicker, stiffer biofilms that are more resistant to antibiotics like ciprofloxacin.

## Contribution

The study reveals that microplastics actively modulate biofilm structure and EPS composition, leading to increased antibiotic tolerance in E. coli.

## Key findings

- MP-biofilms showed 60% cell viability after ciprofloxacin exposure, compared to 24% in G-biofilms and minimal in controls.
- MP-biofilms were seven times thicker and had localized high cell density around microbeads.
- MP-biofilms had a >2.5-fold increase in EPS proteins compared to controls.

## Abstract

Microplastics are increasingly recognized as substrates that facilitate microbial colonization and may contribute to antimicrobial resistance, yet their role in shaping biofilm physiology remains poorly understood. Here, we investigated the antibiotic susceptibility, structural features, mechanical properties, and composition of extracellular polymeric substances (EPS) of Escherichia coli (E. coli) biofilms grown under flow and under identical conditions with three different materials: control (C), glass microbeads (G), and microplastic 10-μm beads (MP). We performed 24h antibiotic susceptibility tests using ciprofloxacin and found significantly enhanced tolerance in MP-biofilms, with approximately 60% of cells remaining viable after exposure to 350 μg/mL, compared to 24% in G-biofilms and minimal survival in controls at lower concentrations of ciprofloxacin (P < 0.0001). Reducing microplastic concentrations ten-fold did not enhance susceptibility, whereas lighter, hollow glass beads generated significantly more susceptible biofilms. MP-biofilms were shown to be nearly seven times thicker than control biofilms and exhibited localized zones of high cell density surrounding the microbeads. We further observed lower creep compliance in MP- and G-biofilms relative to controls, indicating increased stiffness. Finally, we analyzed EPS matrix composition and found that only MP-biofilms displayed substantial enrichment across all EPS components, especially proteins (>2.5-fold increase, P < 0.0001). Together, these results indicate that microplastics can not only serve as favorable surfaces for bacterial attachment and colonization but also actively promote biofilm architectures and biochemical features that confer elevated antibiotic tolerance. Our findings highlight microplastics as contributors to drug-tolerant biofilm microbial communities and reinforce their role as emerging environmental drivers of antimicrobial resistance.