Key Takeaways & Executive Findings
- •• • At pH 3–7, low pH induces inward condensation and protonation of ARP functional groups, reducing negative charge and increasing aggregate size; this pH sensitivity is critical for predicting ARP mobility in acidic soil and groundwater systems. • • In 0.001–0.1 mol·L−1 NaCl, ARP aggregation increases with ionic strength due to double-layer compression; however, Ca2+ (0.001–0.05 mol·L−1) triggers significantly stronger aggregation via cationic bridging, with a threshold effect at 0.01 mol·L−1 CaCl2 where hydrodynamic diameter (dp) and zeta potential shift markedly. • • In the presence of 20 mg·L−1 natural colloids, ARP heteroaggregation is enhanced; at 0.1 mol·L−1 NaCl versus 0.01 mol·L−1 CaCl2, both dp and zeta potential increase, indicating that higher ionic strength compresses the double layer, reduces surface charge, and promotes colloid-ARP aggregation. • • Increasing natural colloid concentration reduces steric hindrance and increases collision frequency, leading to enhanced adsorption or heteroaggregation; this effect is more pronounced with Ca2+ than Na+, underscoring the role of divalent cations in facilitating ARP transport in colloid-rich environments.
Abstract
The excessive and uncontrolled use of antibiotics inevitably leads to their release into natural environments, accelerating the production, occurrence, and transport of resistant bacteria and resistance genes. Among these, antibiotic resistance plasmids (ARPs) pose a significant public health challenge due to their environmental persistence and ability to spread and amplify within microbial communities. This study used the tetC gene-pUC18 plasmid as a model ARP to investigate aggregation behavior in aqueous environments under varying pH (3–7), ionic strength (0.001–0.1 mol·L−1 NaCl and 0.001–0.05 mol·L−1 CaCl2), and in the presence of different concentrations of natural colloids. Results indicate that at low pH, ARP structure condenses inward and functional groups may protonate, reducing negative charge and overall size. Compared to Na+, Ca2+ forms cationic bridges between negatively charged phosphate diester groups, significantly enhancing aggregation. Natural colloids induce heteroaggregation with ARPs, with aggregate size increasing with colloid concentration. This study provides scientific evidence for elucidating ARP behavior in soil and groundwater, crucial for assessing risks to human health and ecosystems and understanding global circulation mechanisms.
1. Introduction
Antibiotic resistance plasmids (ARPs) persist in environmental matrices far longer than their antibiotic counterparts, facilitating horizontal gene transfer across microbial communities and posing a critical threat to public health. While extensive research has addressed antibiotic degradation, the colloidal stability and transport of ARPs in aquatic systems remain poorly understood. Existing commercial water treatment and soil remediation strategies fail to account for the physicochemical factors governing ARP aggregation, leading to inaccurate risk assessments and ineffective containment measures. The bottleneck lies in the lack of systematic data on how pH, ionic composition, and natural colloids influence ARP fate, particularly in complex hydrochemical environments.
This study directly addresses this gap by systematically investigating the aggregation of a model ARP (tetC-pUC18) under controlled pH (3–7), ionic strengths (NaCl and CaCl2), and natural colloid concentrations. By quantifying hydrodynamic diameter and zeta potential, the research delineates the mechanisms—charge neutralization, cationic bridging, and heteroaggregation—that dictate ARP stability. These findings provide a quantitative foundation for predicting ARP mobility in soil and groundwater, enabling more accurate environmental risk assessments and informing the design of intervention strategies to mitigate the spread of antibiotic resistance.
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GUO Yuhang, LIANG Yan, LI Jinyu, LYU Xiaoyan (2026). Aggregation of Antibiotic Resistance Plasmids under Different Hydrochemical Conditions. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024121101
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Frequently Asked Questions
What is the critical ionic strength threshold for Ca2+-induced aggregation of ARPs, and how does it compare to Na+?
In this study, ARP aggregation was significantly enhanced at CaCl2 concentrations as low as 0.01 mol·L−1, whereas NaCl required higher concentrations (up to 0.1 mol·L−1) to achieve comparable effects. This is attributed to Ca2+ forming cationic bridges between phosphate groups, a mechanism absent with monovalent Na+.
How does pH affect the surface charge and aggregation of ARPs, and what are the implications for acidic environments?
At pH 3–7, lower pH leads to protonation of functional groups and inward structural condensation, reducing negative charge and increasing aggregate size. This suggests that in acidic soils or groundwater, ARPs may aggregate more readily, potentially reducing their mobility but also enhancing their persistence and bioavailability.
What is the role of natural colloids in ARP aggregation, and how does ionic strength modulate this interaction?
Natural colloids induce heteroaggregation with ARPs, with aggregate size increasing with colloid concentration. Higher ionic strength (e.g., 0.1 mol·L−1 NaCl vs. 0.01 mol·L−1 CaCl2) compresses the double layer, reducing electrostatic repulsion and promoting colloid-ARP attachment, as evidenced by increased hydrodynamic diameter and zeta potential.
Can the aggregation behavior of ARPs be predicted using classical DLVO theory, or are additional forces involved?
While DLVO theory explains the effects of ionic strength and pH via double-layer compression and charge neutralization, the enhanced aggregation in Ca2+ solutions indicates specific ion interactions (cationic bridging) beyond DLVO. This necessitates incorporating non-DLVO forces for accurate prediction in divalent-rich environments.
What are the practical implications of these findings for assessing ARP transport in soil and groundwater?
The results indicate that in calcium-rich or acidic environments, ARPs are more likely to aggregate and thus may be retained in soil matrices, reducing their vertical transport. Conversely, in low-ionic-strength, neutral-pH conditions, ARPs remain dispersed and can migrate longer distances, increasing the risk of spreading resistance genes to downstream receptors.
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