Tetracyclines are a class of antimicrobials extensively used as human and veterinary medicine, and in livestock production since they were discovered in the 1940s. A large portion of tetracyclines administered to humans and animals are excreted and subsequently released into the environment, where they pose potential risks to ecosystem and human health. There is a growing concern that the presence of antibiotics such as tetracycline at trace levels in the environment is related to the emergence and ever-increasing abundance of antibiotic resistance genes in natural and engineered microbial populations. However, basic knowledge at the molecular scale of bacterial access to tetracyclines present in environmental matrices and expression of antibiotic resistance genes remain nearly unknown. In this study, we used the E. coli MC4100/pTGM whole-cell bioreporter as an effective tool to investigate bioavailability of tetracycline in water and soil to bacteria for expression of antibiotic resistance genes. Our hypothesis was that the speciation of tetracycline dissolved in water and sorption by soil minerals controls the bioavailabilities for bacterial uptake and subsequent activation of antibiotic resistance genes. The results revealed that activation of antibiotic resistance in the E. coli bioreporter responded linearly to intracellular tetracycline concentration. The extent of tetracycline uptake by E. coli was modulated by tetracycline speciation. We have identified that zwitterionic tetracycline as the primary species favorable for bacterial uptake. Geochemical factors such as pH, salt composition and concentration influenced the fractional distributions of tetracycline species in aqueous solution and hence altered uptake by E. coli. In addition, the presence of organic ligands could also alter tetracycline speciation by releasing tetracycline from its metal complexes in aqueous solution. For tetracycline associated with Mg-smectite, desorption of tetracycline from clay to solution was the major exposure pathway for bacterial uptake and subsequent activation of antibiotic resistance in the diluted clay suspensions. In clay film cultivation, clay-sorbed tetracycline was still bioaccessible to E. coli evoking strong expression of antibiotic resistance. Direct contact of the E. coli bioreporters with clay surfaces and further formation of biofilms plausibly facilitated tetracycline transfer to bacteria. Overall, this study greatly advances the fundamental understanding of bioavailability of tetracycline in the environment to bacteria for expression of antibiotic resistance genes.
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