Zhang, YifanThe widespread use of antimicrobials in human and veterinary medicine, as well as in animal production has accelerated the development of drug resistance in a variety of pathogenic bacteria. Listeria monocytogenes and Entercoccus faecium are important Gram-positive pathogens of food safety and public health concern. But their mechanisms of antimicrobial resistance are relatively less clear than those in Gram- negative pathogens. L. monocytogenes (n=167) recovered from deli meats, conventional and organic chicken, and conventional and organic fresh produce samples were characterized by serogrouping, DNA fingerprinting and antimicrobial susceptibility testing. The isolates belonged to five different serogroups. Percentages of resistance to ciprofloxacin, tetracycline, sulfonamide, and nalidixic acid were 1.8%, 9%, 73%, and 100%, respectively. The identification of potential serotype 4b from all food categories, especially from organic chicken products, raised a public health concern, because serotype 4b has been the number one serotype associated with clinical isolates. Multiresistant L. monocytogenes strains were recovered from the food supply, including organic food products, suggesting our food supply may serve as the reservoir for multiresistant L. monocytogenes and the resistance genes. The PFGE and serogroup data also suggest the diverse sources of contamination. E. faecium isolates (n=34), including 33 from seven poultry farms and one from an outpatient in Michigan, were studied by characterizing the quinupristin-dalfopristin (Q/D) resistant plasmids that carried vatE. Hybridization following restriction endonuclease digestion identified five different plasmid types. The vatE-carrying plasmid from the human isolate showed nearly identical hybridization patterns, following restriction endonuclease digestion, to a vatE carrying plasmid from an E. faecium recovered from a chicken farm. This study showed that a heterogeneous group of plasmids harbour vatE in a heterogeneous population of E. faecium . Some of the plasmids were obtained by E. faecium capable of infecting humans. Q/D resistant E. faecium (n=28) with the MICs (minimal inhibitory concentrations) = 32 ug/ml were characterized by gene identification, conjugation, transformation, and in vitro transposon mutagenesis. vatE and ermB are responsible for high streptogramin resistance in most E. faecium isolates from poultry products but that the mechanisms of Q/D resistance among E. faecium isolates from poultry farms remain largely uncharacterized.en-USDissertationAgriculture, Food Science and TechnologyAntimicrobial resistanceListeria monocytogenesEnterococcus faecium