Defense Date


Document Type


Degree Name

Doctor of Philosophy


Microbiology & Immunology

First Advisor

Dr. Gordon Archer


In the past decade, Staphylococcus aureus has developed two distinct vancomycin resistance mechanisms. First, the bacterium is capable of generating a thickened, poorly crosslinked cell wall that creates false targets. These targets cause vancomycin to bind at the periphery of the thickened peptidoglycan, allowing normal cell wall formation to continue at the cell membrane. Second, S. aureus has acquired genes from Enterococcus that encode an alternative stem peptide. The genes, known as van genes, alter the target of vancomycin, rendering vancomycin treatment ineffectual.In this work, we attempted to further characterize both mechanisms of vancomycin resistance. First, a potential link between up-regulated purine biosynthesis and increased vancomycin resistance due to a thickened cell wall was examined. Despite exploration of multiple mechanisms to increase purine levels within the cell, increased purine synthesis did not provide S. aureus with any advantage in the presence of vancomycin. However, during the investigation, purine biosynthesis in S. aureus was further characterized by confirming purr as the repressor of the purine pathway and demonstrating its sensitivity to mutation.Next, the relationship between homotypic oxacillin resistance and increased vancomycin resistance in the absence of the van genes was investigated. Vancomycin passage of two heterotypic methicillin resistant S. aureus (MRSA) caused these strains to convert to homotypic oxacillin resistance in the absence of oxacillin exposure. Additionally, conversion of heterotypic oxacillin resistant strains to homotypy by oxacillin passage increased strain survival in vancomycin. The SOS response was examined as the possible link between conversion to homotypic oxacillin resistance and increasing vancomycin resistance due to a thickened cell wall. The current study, however, detected no induction of the SOS response during vancomycin exposure.Lastly, the relationship between oxacillin resistance and vancomycin resistance due to the acquisition of the van genes was examined. In vitro and in vivo methods were utilized to determine the effectiveness of a combination of β-lactam antibiotics and vancomycin to treat vancomycin resistant S. aureus (VRSA) infections. Combination therapy provided a significant advantage over untreated control or either antibiotic alone in the rabbit model of endocarditis.


© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

June 2008