Date of Award
Doctor of Philosophy (PhD)
Antimicrobial chemicals in consumer personal care products have been found to increase antibiotic resistance in pure culture studies. Although many studies focus on antibiotic resistance development pertinent to medical scenarios, resistance developed in natural and engineered environments might be significant and has become an emerging concern for human health. This dissertation focuses on the antimicrobial chemicals triclosan and triclocarban. These compounds are distinctly different from antibiotics and are used in products like soaps that are labelled as “antibacterial”. Municipal wastewater treatment plants receive triclocarban and triclosan loads higher than most contaminants of emerging concern because they are frequently used in consumer products and then discharged into the sewerage system. This research specifically focused on the impact of triclosan and triclocarban in lab-scale anaerobic digesters and investigated how they influenced digester function, the relative abundance of resistance genes, microbial community structure, and cross-resistance to antibiotics. Lab-scale anaerobic digesters were operated for 180 days and loaded with concentrations of triclocarban or triclosan ranging zero to inhibitory concentrations. Both triclosan and triclocarban selected for mexB, a gene that confers multidrug resistance in bacteria, at environmentally relevant concentrations. This is the first research to demonstrate that triclocarban can select for a multidrug resistance gene in anaerobic digesters. Relatively higher concentrations of these chemicals inhibited function in anaerobic digesters and further selected for some resistance genes and against others. The functional inhibition was not reversible when chemicals were removed. When these chemicals were removed from functioning digesters the mexB concentrations were no longer different from the control digesters suggesting that a decrease in consumer usage could have impacts on environmental antibiotic resistance. At higher concentrations of triclosan, and all concentrations of triclocarban, digester microbial community structures irreversibly shifted away from the control. In a separate set of experiments, addition of these antimicrobials altered how anaerobic digester microbial communities responded to the presence of three other antibiotics. Triclosan-amended communities had increased resistance to ciprofloxacin; triclocarban-communities were more sensitive to tetracycline and chloramphenicol. This research demonstrates that antimicrobials should be considered along with antibiotics when determining the role of chemical stress on the proliferation of antibiotic resistance.