ABSTRACT Title of Dissertation: CHARACTERIZATION AND MOLECULAR MECHANISMS OF ANTIMICROBIAL RESISTANCE

The emergence of antimicrobial resistance bacteria in both the medical and agricultural fields has become a serious problem worldwide. The use of antimicrobial agents in animals for prophylaxis and growth promotion favors the selection of antimicrobial resistance in bacteria. These resistant bacteria may subsequently transmit to humans through food chain or human-animal interactions. The objectives of this study were to characterize and determine the molecular mechanisms of antimicrobial resistance in Salmonella and Escherichia coli. E. coli isolates recovered from diseased piglets and chicken in China were characterized for the antimicrobial resistance phenotype and the presence of mutation in their gyrA, gyrB, parC and parE genes. Results indicated that multiple- antimicrobial-resistant E. coli, including fluoroquinolone-resistant variants, are commonly present among diseased swine and chickens in China. DNA sequencing revealed that double gyrA mutations coupled with parC mutation conferred high-level resistance to fluoroquinolones. In addition, Salmonella isolates recovered from retail meats in the United States and China were characterized for antimicrobial resistance genotypes and the horizontal transfer o