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Published on: July 11, 2012
Size- and surface charge-dependent hormetic effects of microplastics on bacterial resistance and their interactive
Hongyan Shen1, Mingru Yang1, Kangnian Yin1
1School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China.
Abstract:
The facilitation of microplastics (MPs) on bacterial resistance has attracted wide concern, due to the widespread presence of MPs in environmental media and their ubiquitous contact with bacteria strains. Furthermore, MPs possibly co-exist with antibiotics to trigger combined stress on bacterial survival. Therefore, it is significant to reveal the dose-responses of MPs and MP-antibiotic mixtures on bacterial endogenous and exogenous resistance. In this study, 0.1 and 5 μm polystyrenes with no surface functionalization (PS-NF, no charge), surface functionalized with amino groups (PS-NH2, positive charge) and carboxyl groups (PS-COOH, negative charge) were selected as the test MPs, and norfloxacin (NOR) was set as the representative of antibiotics. It was found that six types of PS all inhibited the growth of Escherichia coli (E. coli) but induced hormetic dose-responses on the mutation frequency (MF) and conjugative transfer frequency (CTF) of RP4 plasmid in E. coli. Moreover, these hormetic effects exhibited size- and surface charge-dependent features, where 0.1 μm PS-NH2 (100 mg/L) triggered the maximum stimulatory rates on MF (363.63 %) and CTF (74.80 %). The hormetic phenomena of MF and CTF were also observed in the treatments of PS-NOR mixtures, which varied with the particle size and surface charge of PS. In addition, the interactive effects between PS and NOR indicated that the co-existence of PS and NOR might trigger greater resistance risk than the single pollutants. Mechanistic exploration demonstrated that the increase of cellular reactive oxygen species and the variation of cell membrane permeability participated in the hormetic effects of PS and PS-NOR mixtures on bacterial resistance. This study provides new insights into the individual effects of MPs and the combined effects of MP-antibiotic mixtures on bacterial resistance, which will promote the development of environmental risk assessment of MPs from the perspective of bacterial resistance.
Insights
Microplastics (MPs) and antibiotics can increase bacterial resistance. This study found that microplastics, especially smaller, positively charged ones, and their mixtures with norfloxacin (NOR) induced hormetic dose-responses on bacterial mutation and plasmid transfer frequencies.
Area of Science:
- Environmental Science
- Microbiology
- Toxicology
Background:
- Microplastics (MPs) are pervasive environmental contaminants. MPs can interact with bacteria, potentially influencing antibiotic resistance. Co-exposure to MPs and antibiotics may pose combined risks to bacterial survival and resistance.
- Understanding the dose-response relationship of MPs and MP-antibiotic mixtures on bacterial resistance is crucial for environmental risk assessment.
Purpose of the Study:
- To investigate the dose-responses of different microplastic (MP) types and MP-antibiotic mixtures on bacterial endogenous and exogenous resistance.
- To elucidate the influence of MP size and surface charge on bacterial mutation frequency (MF) and conjugative transfer frequency (CTF).
- To explore the interactive effects of MPs and antibiotics on bacterial resistance and the underlying mechanisms.
Main Methods:
- Tested six types of polystyrene MPs (0.1 and 5 μm; non-functionalized, amino-functionalized, carboxyl-functionalized) and norfloxacin (NOR) on Escherichia coli (E. coli).
- Assessed the effects of MPs and MP-NOR mixtures on E. coli growth, MF, and CTF using dose-response experiments.
- Analyzed size- and surface charge-dependent effects of MPs and their mixtures.
- Investigated mechanisms including reactive oxygen species (ROS) production and cell membrane permeability.
Main Results:
- All tested MPs inhibited E. coli growth but induced hormetic dose-responses on MF and CTF, indicating increased bacterial resistance at certain concentrations.
- Hormetic effects were size- and surface charge-dependent, with 0.1 μm amino-functionalized PS showing the maximum stimulatory rates on MF and CTF.
- MP-NOR mixtures also exhibited hormetic effects on MF and CTF, with interactive effects suggesting greater resistance risk compared to single pollutants.
- Increased ROS and altered cell membrane permeability were identified as mechanisms contributing to these hormetic effects.
Conclusions:
- Microplastics, particularly smaller and positively charged ones, can stimulate bacterial mutation and plasmid transfer frequencies, contributing to antibiotic resistance.
- The combination of microplastics and antibiotics may exacerbate bacterial resistance risks.
- Mechanisms involving oxidative stress and altered membrane permeability are involved in microplastic-induced bacterial resistance.
- This research provides critical insights for evaluating the environmental risk of microplastics concerning bacterial resistance.
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