Date of Award

Summer 8-1-2015

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

Committee Chair

Mohamed O. Elasri

Committee Chair Department

Biological Sciences

Committee Member 2

Glenmore Shearer

Committee Member 2 Department

Biological Sciences

Committee Member 3

Yanlin Guo

Committee Member 3 Department

Biological Sciences

Committee Member 4

Shahid Karim

Committee Member 4 Department

Biological Sciences

Committee Member 5

Fengwei Bai

Committee Member 5 Department

Biological Sciences


Staphylococcus aureus is the predominant cause of public health problems around the world. Vancomycin has been an important antibiotic against Methicillin Resistant Staphylococcus aureus (MRSA) infections. However, Vancomycin Intermediate S. aureus (VISA) strains have been reported. These strains are characterized by thick cell walls, reduced autolytic rate, reduced PBP4 activity, and increased amount of D-Ala-D-Ala termini in the cell wall. In this study, we show that the msaABCR operon regulates vancomycin resistance in two clinical VISA strains. Deletion of the msaABCR operon in strains Mu50 and HIP6297 resulted in a significant decrease in the minimum inhibitory concentration (MIC) for vancomycin. Transmission Electron Microscopic analysis showed a 50% decrease in the cell wall thickness in the mutants relative to wild types. The msaABCR mutant of Mu50 but not HIP6297 showed a 3-fold up-regulation of pbp4. Up-regulation of pbp4 was confirmed by fluorescent penicillin–binding assay, which showed that the mutant has a 6-fold higher amount of PBP4 protein relative to wild type. Mu50 msaABCR mutant also showed a 6-fold down-regulation of stress response regulator sigB. On the other hand, the msaABCR mutant of HIP6297 showed a 4-fold down-regulation of mprF. Collectively, my data show that the msaABCR is required for maintaining the thickness of cell wall and vancomycin resistance in two clinical VISA strains Mu50 and HIP6297; however, the mechanism and regulation by the msaABCR operon in the genetic level are strain dependent.

Staphylococcus aureus is also the predominant cause of bacteremia worldwide. The health hazard burden caused by this species is severely exacerbated by worldwide dissemination of clones resistant to beta-lactam antibiotics in hospitals and communities. The distribution of MRSA clones is dynamic and tends to be geographically unique. The purpose of the second part of my study is to determine the molecular characteristics and antibiotic resistance properties of the MRSA isolates causing bacteremia in a major hospital in south Mississippi, USA. This study allows us to associate specific clonal types with bacteremia isolates and detect outbreak by any particular type in south Mississippi.