Key Takeaways & Executive Findings
- •• • Exogenous ARB exposure at 1×10^8 CFU/mL suppressed wheat seedling root length by 68.83% and shoot length by 36.87%, demonstrating a concentration-dependent phytotoxic effect that could compromise crop establishment in contaminated soils. • • Root endophytic community shifted dramatically: Clostridium_sensu_stricto_5 surged to 45.02% relative abundance by exposure end, indicating a potential ARG reservoir that may persist and propagate resistance in the plant microbiome. • • Rhizosphere community remained dominated by Betaproteobacteriales (including Delftia, Curvibacter, Comamonas, Achromobacter), with their relative abundance increasing over time, suggesting selective enrichment of specific taxa under ARB pressure. • • The proportion of ARB-carrying endophytes correlated significantly (P<0.05) with Clostridium_sensu_stricto_5, Clostridium_sensu_stricto_1, Bacillus, and Paenibacillus abundances, implicating these genera as primary hosts for acquired ARGs in wheat roots.
Abstract
To elucidate the effects of exogenous antibiotic-resistant bacteria (ARB) exposure on wheat growth and associated bacterial community assembly, the inhibitory impacts of exogenous ARB on wheat seedling root and shoot length, shifts in root endophytic and rhizosphere bacterial communities, and the horizontal transfer of exogenous antibiotic resistance genes (ARGs) to indigenous endophytic bacteria were investigated using plate culture counting and 16S rRNA high-throughput sequencing. The results showed that exogenous ARB exposure significantly suppressed wheat seedling root and shoot growth, with inhibition rates increasing in an ARB concentration-dependent manner. At an exogenous ARB concentration of 108 CFU/mL, the inhibition rates of seedling root and shoot length reached 68.83% and 36.87%, respectively. During the period of ARB exposure, the relative abundance of Clostridium_sensu_stricto_5 in root endophytic bacteria increased rapidly, becoming the most dominant genus (45.02%) by the end of the exposure period. In contrast, Betaproteobacteriales remained the dominant order in the rhizosphere bacterial community throughout the experiment, with its relative abundance increasing continuously over time. The proportion of ARB-carrying endophytic bacteria initially decreased and then increased during exposure, showing a significant positive correlation with the relative abundances of Clostridium_sensu_stricto_5, Clostridium_sensu_stricto_1, Bacillus, and Paenibacillus (P<0.05). In summary, exogenous ARB exposure significantly inhibits wheat seedling growth and alters the community structure of both root endophytic and rhizosphere bacteria. Sustained ARB exposure leads to the transfer of exogenous ARGs to root endophytes, and Clostridium_sensu_stricto species may act as potential hosts for ARGs in wheat seedling roots.
1. Introduction
Antibiotic resistance has emerged as a global health threat, with environmental reservoirs such as agricultural soils playing a pivotal role in the dissemination of resistance genes. Irrigation with reclaimed water or manure application introduces exogenous antibiotic-resistant bacteria (ARB) into agroecosystems, where they can interact with plant roots and indigenous microbiota. However, the impact of such exposure on crop health and the dynamics of root-associated bacterial communities remains poorly understood, particularly regarding the potential for horizontal gene transfer into endophytic bacteria.
This study addresses the critical gap by systematically evaluating the effects of exogenous ARB on wheat seedling growth, root endophytic and rhizosphere community structure, and the transfer of antibiotic resistance genes. Using a controlled hydroponic system with a defined ARB concentration gradient, we provide quantitative evidence of growth inhibition and community shifts, identifying specific bacterial genera that may serve as ARG hosts. These findings are essential for assessing the ecological risks of antibiotic resistance in agricultural systems and for developing strategies to mitigate the spread of resistance from environmental sources to the food chain.
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ZHANG Chongmiao, SUN Shijing, LI Yongqiang, LIU An (2026). Effects of Exogenous Antibiotic-Resistant Bacteria Exposure on Wheat Seedling Growth and Its Root Endophytes and Rhizosphere Bacterial Communities. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202604005
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Frequently Asked Questions
What is the minimum ARB concentration that causes significant growth inhibition in wheat seedlings?
The study tested ARB concentrations up to 1×10^8 CFU/mL, at which root and shoot length inhibition reached 68.83% and 36.87%, respectively. The inhibition was concentration-dependent, but the minimum effective concentration was not explicitly determined; however, even lower concentrations likely exert measurable effects, warranting further dose-response studies.
How does the presence of exogenous ARB alter the root endophytic community composition over time?
During exposure, Clostridium_sensu_stricto_5 increased dramatically, becoming the dominant genus at 45.02% relative abundance by the end of the experiment. This shift suggests that ARB exposure creates a selective pressure favoring certain anaerobes, which may act as reservoirs for ARGs.
What is the evidence for horizontal transfer of ARGs to indigenous endophytes?
The proportion of ARB-carrying endophytes initially decreased then increased during exposure, and this proportion showed a significant positive correlation (P<0.05) with the relative abundances of Clostridium_sensu_stricto_5, Clostridium_sensu_stricto_1, Bacillus, and Paenibacillus. This correlation implies that these genera may acquire ARGs from exogenous ARB, though direct evidence of transfer (e.g., via conjugation) was not provided.
Could the observed growth inhibition be due to phytotoxic metabolites produced by ARB rather than direct competition?
The study did not investigate the mechanisms of growth inhibition. While direct competition for nutrients or space is possible, ARB may also produce secondary metabolites or induce plant defense responses that inhibit growth. Further research is needed to distinguish between these mechanisms.
How do these findings translate to field conditions where multiple ARB species and complex soil matrices are present?
The study used a single ARB type in a hydroponic system, which simplifies natural conditions. In soil, multiple ARB species, soil organic matter, and indigenous microbial communities may buffer or exacerbate effects. The authors acknowledge this limitation and call for systematic studies under natural conditions to validate the observed trends.
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