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Title:Investigating the role of nonantibiotic chemical contaminants in the evolution of antibiotic resistance
Author(s):Xing, Yue
Director of Research:Men, Yujie
Doctoral Committee Chair(s):Men, Yujie
Doctoral Committee Member(s):Liu, Wen-Tso; Nguyen, Thanh H; Whitaker, Rachel
Department / Program:Civil & Environmental Eng
Discipline:Environ Engr in Civil Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):antibiotic resistance
selective pressure
emerging organic contaminants
Escherichia coli
evolutionary trajectory
environmental factors
Abstract:Antibiotic resistance, the ability of bacteria to resist the effects of antibiotic drugs, has now become a global threat to public health. Every year around 700,000 people die due to the antibiotic-resistant bacterial infections, and this number is even predicted to reach as many as 10 million by the year of 2050 if no actions are taken. Not only in clinical settings, antibiotic resistant bacteria and resistance genes are frequently detected in environmental compartments, such as wastewater treatment systems and animal husbandry facilities. Considering that the antibiotic resistance developed in those environments may re-enter water cycle and food chain, there could be potential transmissions of antibiotic resistance from the environmental hotspots to medical settings. Thus, it is urgent to control the emergence and spread of antibiotic resistance in the environments. The resistance could be acquired either by genetic mutations or horizontal transfer of resistance genes. These acquisition processes could be promoted by selective pressures, in which antibiotics are the long-standing study focus. However, a variety of chemical contaminants are present in natural and built environments. It is unclear how these nonantibiotic contaminants would affect the selection of antibiotic resistance. Aimed to answer this question, my work systematically investigated the role of nonantibiotic chemical contaminants in the evolution of antibiotic resistance. I started the investigation of the occurrence of chemical contaminants in wastewater. The influent and effluent samples from two local wastewater treatment plants in Urbana-Champaign were analyzed by HPLC coupled with high-resolution mass spectrometry. A suspect screening method was applied with a comprehensive suspect list of over 1000 chemicals in 5 categories: pharmaceuticals, personal care products, pesticides, industrial chemical, and metabolites. Paracetamol, benzotriazole, gabapentin, and DEET were detected in the influent with high abundance. The results also revealed that many chemical contaminants, such as fexofenadine, and prometon, had low removal efficiencies. This study suggests the prevalence of chemical contaminants and their coexistence in the wastewater. I then investigated the effects of pesticide mixture, which are nonantibiotic chemical contaminants and co-occur with antibiotics in the environments, on the development of antibiotic resistant bacteria. Long-term exposure experiments under pesticide exposure conditions were conducted in the Escherichia coli K-12 strain. The results reveal that (1) the exposure to pesticides (mg/L) alone lead to the emergence of mutants with significantly higher resistance to streptomycin; (2) the coexposure to pesticides (in μg/L) and a subinhibitory level (in high μg/L) of the antibiotic, ampicillin, synergistically stimulated the selection of ampicillin resistance and the cross-resistance to other antibiotics. Using genomic and transcriptome analysis, I have identified distinct and diversified genetic mutations in resistant mutants selected from the exposure conditions with pesticides, which likely caused a holistic transcriptional response (e.g., biofilm formation, oxidative stress defense) and led to increased resistance. I further explored whether the synergy of pesticides and ampicillin could be applied to other antibiotics and how pesticides as the co-stressors affect the evolutionary trajectory toward antibiotic resistance. The coexposure to pesticides and another antibiotic, streptomycin, substantially changed the phenotypic, genotypic, and fitness trajectories, resulting in much stronger resistance (> 15-fold increase) of the populations. A succession pattern from the mildly resistant mutants with off-target mutations to the strongly resistant mutants with target modification mutations was found in the coexposed bacterial populations. Moreover, the relative fitness between different genotypes of cells has driven the evolution toward strong resistance, and the strongly resistant mutants exhibited higher fitness than the mildly resistant mutants even without the selective pressure. These findings depict the complete process of evolution with pesticide co-stressors, implying that the removal of selective pressure alone may not reverse strong resistance at a certain evolutionary period. In addition, the bacterial specificity to the synergy of pesticides and streptomycin was studied, and the results showed that the synergistic effect of pesticides and streptomycin could also be exerted in the E. coli O157: H7, but not the other tested strains (i.e., Pseudomonas putida and Staphylococcus epidermidis), suggesting the specificity of E. coli strains. Finally, I investigated how physicochemical parameters, including temperature, pH, salinity, and different nutrient elements would affect the growth of resistant and susceptible bacteria. The results showed that several physicochemical parameters (e.g., pH, KCl, Fe-limited condition) could affect the relative fitness of susceptible bacteria and resistant mutants, both with and without antibiotic stress, suggesting that other environmental factors could also influence the maintenance and selection of antibiotic resistance. Overall, my work elucidates that pesticides could induce de novo resistant mutants and drive the evolutionary trajectories toward higher resistance in the bacterial populations. These results highlight the overlooked role of nonantibiotic chemical contaminants in promoting antibiotic resistance, which provide a better understanding of antibiotic resistance evolution in the environments and help making better mitigation strategies against antibiotic resistance.
Issue Date:2021-07-11
Rights Information:Copyright 2021 Yue Xing
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08

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