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Mechanisms of 6PPD Affecting the Conjugative Transfer of Antibiotic Resistance Genes
Summary
Tiny amounts of a chemical called 6PPD—which comes from car tires and washes into soil through rain and dust—may help bacteria pass antibiotic-resistant genes to each other more easily, according to lab tests using low, real-world levels of the substance. This matters because tire particles are everywhere in our environment, and if they're quietly boosting the spread of antibiotic resistance in agricultural soils, that could make it harder to treat infections down the line. The study only found a link (not proof) that the chemical stresses bacteria cells in a way that encourages this gene-swapping, so more research is needed to confirm exactly
Within the “One Health” framework, the dissemination of antibiotic resistance genes (ARGs) in agricultural soils has drawn considerable attention. A novel contaminant—the tire antioxidant additive N- (1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD)—can enter agricultural soils through tire wear particles and their leachates. As an emerging organic pollutant, the impact of 6PPD on horizontal gene transfer (HGT) of ARGs remains underexplored. To evaluate its effect on interspecies conjugation under conditions relevant to agricultural soil environments, an intergeneric mating system was established simulating oligotrophic soil conditions, employing Pseudomonas putida KT2440 as the donor and Escherichia coli K12 as the recipient (hereafter referred to as the donor and recipient strains, respectively). Using environmentally relevant concentrations (0.5~20 μg L⁻¹), this work systematically examined how 6PPD influences the RP4 plasmid-mediated conjugative transfer from P. Putida KT2440 to E. coli K12, and further elucidated the underlying molecular mechanism by quantifying intracellular reactive oxygen species (ROS) levels in both mating partners. The results demonstrate that 6PPD significantly promotes conjugation in a dose-dependent manner, an effect primarily achieved via the induction of intracellular ROS accumulation within the donor and recipient cells. It must be noted, however, that the present study has only established a correlative relationship between ROS buildup and enhanced conjugative transfer; a causal link still awaits confirmation through additional experiments, such as those employing ROS scavengers. Collectively, these findings offer experimental evidence for identifying and assessing the risk that tire-derived contaminants pose in driving the spread of environmental antibiotic resistance in soils..