Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Nutrition and Food Science

First Advisor

Yifan Zhang

Abstract

Antimicrobial resistance in Staphylococcus spp. is a worldwide epidemic concern in hospital and community settings. Food animals and retail meat are important reservoirs of these pathogens that can pose potential threat to humans. In this dissertation, we aimed to investigate the molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Coagulase-negative Staphylococci (MRCoNS) in food animals and retail meat to provide insight into the role of agricultural environment in transmitting bacteria of human clinical significance. Furthermore, the potential application of phytochemicals as antimicrobials and antimicrobial adjuvants to control MRSA infections was explored.

CoNS recovered from food animals were characterized by antimicrobial susceptibility testing, resistance gene identification and conjugation. Staphylococcus lentus, Staphylococcus sciuri, Staphylococcus xylosus and Staphylococcus haemolyticus accounted for over 96% of the isolates. Resistance to macrolides, tetracyclines, and Q/D, in addition to Β-lactam resistance was observed with 54% isolates classified as multi-drug resistant. tet(M) was conjugatively transferable from 10 tetracycline-resistant CoNS to other commensals like Enterococcus faecalis by conjugation.

S. aureus was recovered from 65 of 289 meat samples, including 6 samples carrying MRSA. All MRSA isolates were USA 300, the most common community-associated MRSA clone. They exhibited similar molecular profiles (SCCmecIV, ST8, pvl positive and agrI) by various sub typing methods, except for spa typing, which identified 2 distinct spa types, t008 and t2031. Multiple CoNS species, including those that have been speculated as ancestral species of mec(A), such as S. sciuri and S. fleuretti, were prevalent in retail meat. From our SCCmec typing experiments, we identified SCCmec types IV and V in MRCoNS which has been previously found in MRSA from meat.

Most of the phytochemical emulsions demonstrated antimicrobial activity against MRSA. Curcumin, t-cinnamaldehyde and garcinol (MIC: 4-16μg/ml, 312.5μl/L and 2μg/ml) were highly effective in inhibition, whereas, t-cinnamaldehyde and curcumin combinations showed synergistic or additive effect with commercial antimicrobials (cefoxitin, tetracycline and erythromycin) against MRSA.

In conclusion, food animals and retail meat provide a diverse reservoir of antimicrobial-resistant Staphylococcus spp. Multidrug resistance is common in CoNS in animals. The presence of MRSA and MRCoNS in meats may pose potential threat of infection to individuals who handle the meat. The molecular composition of MRCoNS and MSSA strains clearly represent genetic background favorable for the emergence of MRSA. Antimicrobial nano-emulsions of phytochemicals and their combinations with commercial antimicrobials offer alternatives to control MRSA infections.

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