Microplastics: a growing concern
Microplastics (MPs), the degradation products of larger plastics, are tiny particles typically smaller than 5 mm. Along with nanoplastics, they are widely present in marine and terrestrial ecosystems. Because of their small size, they are easily ingested by various species, from zooplankton to fish, potentially accumulating in the food chain and causing ecological harm and environmental pollution.
The large surface area of microplastics is ideal for the deposition of toxins, heavy metals, and biological molecules. Furthermore, the net-neutral charge on the surface of MPs attracts microorganisms, offering an excellent niche for their accumulation through the formation of biofilms. The unique microenvironment of a biofilm, where microorganisms are embedded in a matrix of extracellular polymeric substances, protects and facilitates their survival in harsh environmental conditions.
The unique morphology of microplastics is also ideal for the attachment of antibiotics, which end up in aquatic bodies through agricultural runoff or improper disposal of antibiotics in wastewater. Antibiotics either absorb into or partition within the microplastic matrix. Such sorption of sub-inhibitory concentrations of antibiotics is linked to the growth of antibiotic-resistant bacteria (ARB)—those which have become resistant to antibiotics that were previously efficient in inhibiting their growth.
As previously mentioned, heavy metals are also known to deposit on microplastics. Heavy metals, like Pb, Cd, and Hg, are known to have harmful effects on bacteria. Bacteria are known to evolve and become metal-resistant even before antibiotics were invented. It is now believed that such metal resistance has shaped the evolutionary trajectory of antibiotic resistance. It was found that the co-adsorbed heavy metals, in sub-inhibitory concentrations, also provide synergy to the selection of antibiotic resistance genes (ARGs).
Antimicrobial resistance have become a leading cause of death in the 21st century, and the death rate is only increasing, prompting the World Health Organization to declare antimicrobial resistance a global threat. With microplastics accentuating this threat, it becomes imperative to understand the relationship between MPs and antibiotic resistance.
How microplastics help in transferring ARGs
It begins with eco-corona formation, where microplastics accumulate organic and inorganic substances on their surfaces. This occurs via
- Electrostatic interactions: attracting charged molecules
- Hydrophobic interactions: binding non-polar substances
- Pore-filling: trapping pollutants in surface cracks
The eco-corona exists in two forms:
- A loosely attached “soft corona” with rapidly exchanging molecules
- A tightly bound “hard corona” that alters the microplastic’s structure. This layer modifies environmental behavior by increasing pollutant adsorption, enabling microbial attachment through surface charge changes, and providing nutrients for bacteria.
Further interactions between MPs and bacteria can be explained through the formation of the plastisphere. Here, the initial reversible bacterial adhesion transitions to irreversible binding as biofilms form and bacteria lose mobility.
As the biofilm matures, it forms a complex three-dimensional structure, creating a microenvironment that is rich in nutrients and protected from external stresses. Within this structure, microbial cells are in close proximity to each other, facilitating intense interactions and communication through mechanisms like quorum sensing (cell-density-dependent gene regulation), co-selection (simultaneous resistance to antibiotics and metals), and mobile genetic elements (plasmids or transposons carrying multiple ARGs).
This proximity is also crucial for horizontal gene transfer (HGT)—the movement of genetic material, including ARGs, between bacteria. The plastisphere thus becomes a hotspot for the exchange of genetic material, especially genes that confer resistance to antibiotics and other environmental stressors.
One of the defining features of the plastisphere is its ability to concentrate ARB far more than the surrounding environment. Studies have shown that microplastics can harbor hundreds to thousands of times more ARB than equivalent volumes of water or soil. This is due to several factors: the biofilm structure provides protection and stability, the close cell proximity increases opportunities for gene transfer, and the presence of co-adsorbed pollutants (such as heavy metals and antibiotics) exerts selective pressure that favors resistant strains.
It has been found that sub-inhibitory concentrations of heavy metals act in synergy with antibiotics in the selection of ARGs by enhancing the HGT of plasmid-mediated antibiotic resistance among bacteria. Elevated selective pressure from heavy metals also enhances the selection of de novo mutations, promoting the clonal expansion of resistant cells and accelerating the HGT of ARGs.
Over time, biofilms may disperse, releasing bacteria—including those carrying antibiotic resistance genes—back into the environment. This process can spread resistance across different habitats, increasing the risk of resistant infections in both natural and human-impacted ecosystems.
Conclusion
Microplastics don’t just pose ecological concerns; they also serve as crucial niches for the transfer of antibiotic-resistant genes, with the consequent generation and dispersion of antibiotic-resistant bacteria. This threat is further exacerbated by the synergistic effects of heavy metals deposited on their surfaces. This complex challenge calls for a multi-faceted approach, requiring coordinated efforts across scientific, industrial, medical, and policy sectors to protect both human health and the environment. This necessitates a robust One Health framework. Firstly, we urgently need effective waste management, environmental remediation, and the reduction of single-use plastics. Secondly, comprehensive strategies are needed to control industrial effluents, agricultural runoff, mining waste, and other diffuse sources, ensuring that toxic heavy metals don’t enter our water bodies and accumulate in the environment. Lastly, and most crucially, combating the threat of antibiotic resistance demands responsible antibiotic stewardship, including reducing its overuse in human medicine and agriculture, and ensuring proper disposal to prevent environmental contamination.
Reference
1. Siddique, A., Hubab, M., Rasheela, A. R. P., et al. (2025). Microplastics and their role in the emergence of antibiotic resistance in bacteria as a threat for the environment. Environmental Chemistry and Ecotoxicology, 7, 614-622.
2. Balta, I., Lemon, J., Gadaj, A., et al. (2025). The interplay between antimicrobial resistance, heavy metal pollution, and the role of microplastics. Frontiers in Microbiology, 16, 1550587.