By tracking how heavy metals, microbial communities, and mobile genetic elements interact in agricultural soils, the study uncovers a hidden pathway through which resistance traits proliferate and potentially move from farms into the human food chain.
Globally, antibiotic resistance is responsible for hundreds of thousands of deaths each year, driven not only by medical misuse but also by agricultural practices. Livestock production relies heavily on antibiotics and metal-based feed additives such as copper and zinc, most of which are excreted unmetabolized. When manure is applied to cropland as fertilizer, residual antibiotics, heavy metals, and resistance genes are introduced into soils. These contaminants exert selective pressure on microbes and promote horizontal gene transfer, allowing resistance traits to persist, multiply, and spread. Despite growing concern, comparative data across different livestock systems and direct assessments of human health risk in surrounding environments have remained limited.
A study (DOI:10.48130/biocontam-0025-0007) published in Biocontaminant on 18 November 2025 by Huijun Ding’s team, Nanchang University, reveals that heavy metals indirectly amplify the environmental and human health risks of antibiotic resistance by mobilizing resistance genes through mobile genetic elements, demonstrating that even seemingly low-risk manure can drive high-risk ARG dissemination in agricultural soils.
Using a comparative field-sampling approach around representative pig and chicken farms, this study quantified heavy metals in manure, adjacent vegetable soils, and farm waters, profiled ARGs and mobile genetic elements (MGEs) using quantitative methods, and characterized microbial communities through 16S rRNA gene sequencing, with redundancy analysis, network analysis, and partial least squares pathway modeling applied to link metals, microbes, MGEs, and ARG-related risks. The results showed pronounced zinc enrichment in organic fertilizers, reaching 2.2-fold above background in pig manure (P3) and 2.1-fold in dried chicken manure (C3), with secondary Zn accumulation in nearby pig-farm vegetable soils (P2), while copper followed a similar pattern and peaked in pig manure, indicating feed-additive inputs and more efficient metal dispersal from pig systems, likely driven by wetter slurry runoff. Cadmium levels largely reflected local geological background rather than farming inputs, lead remained near background values, and copper declined sharply from wastewater influent (P4) to oxidation-pond effluent (P5), demonstrating effective treatment. ARG and MGE analyses revealed strong spatial heterogeneity: pig manure (P3) and pig wastewater influent (P4) contained the richest ARG subtype pools, whereas dried chicken manure (C3) harbored very few subtypes but exhibited high ARG abundance dominated by the aminoglycoside resistance gene aadA1, suggesting desiccation selects for a limited yet abundant resistance repertoire. Oxidation-pond treatment reduced ARG abundance by approximately 99%, while chicken manure–amended vegetable soils (C2) showed both markedly elevated ARG abundance and the highest human health risk index, exceeding pig-manure–amended soils and accumulating tetracycline, sulfonamide, and multidrug resistance genes absent from the original fertilizer. Microbial community analyses indicated clustering by sample type rather than farm origin, with a sharp decline in Firmicutes accompanying ARG removal during wastewater treatment, implicating this phylum as a key ARG host. Integrated multivariate and pathway analyses identified zinc, Firmicutes/Chloroflexi, and integron- and transposase-associated MGEs as dominant drivers, demonstrating that heavy metals indirectly promote ARG dissemination primarily by enriching MGEs rather than through direct selection or broad shifts in microbial community structure.
The study challenges the assumption that organic fertilizers are inherently safe. Even dried or “low-risk” manures can trigger substantial ARG dissemination once applied to soils, posing risks to crops and, potentially, consumers. These findings underscore the need for improved manure treatment, stricter control of heavy metal inputs, and routine monitoring of resistance genes in agricultural environments.
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References
DOI
10.48130/biocontam-0025-0007
Original Source URL
https://doi.org/10.48130/biocontam-0025-0007
Funding information
This study was supported by the Jiangxi Provincial Outstanding Youth Foundation Project (20224ACB214013) and the Key Project of Jiangxi Provincial Natural Science Foundation (20232ACB203023).
About Biocontaminant
Biocontaminant is a multidisciplinary platform dedicated to advancing fundamental and applied research on biological contaminants across diverse environments and systems. The journal serves as an innovative, efficient, and professional forum for global researchers to disseminate findings in this rapidly evolving field.