Access Type

Open Access Thesis

Date of Award

January 2016

Degree Type


Degree Name



Pharmaceutical Sciences

First Advisor

Michael J. Rybak





August 2106

Advisor: Dr. Michael J. Rybak

Major: Pharmaceutical sciences

Degree: Masters of Science

Objective: Development of resistance in S. aureus has been a big concern in the treatment of infections caused by the organism. The line of therapy used today has a disadvantage of slowly developing cross-resistance for antibiotics such as DAP secondary to prior VAN exposure. Therefore, alternative therapy is needed to prevent the emergence of VAN resistance and cross-resistance to other antibiotics. The primary aim of this experiment is first: to prove that sub-optimal VAN exposure in S. aureus will lead to development of VISA. Secondly, VAN in combination with cefazolin (CFZ) will prevent the development of VISA, regardless of sub-therapeutic vancomycin exposure. In addition to this we will evaluate if the combination of vancomycin and CFZ is synergistic against various phenotypes of MRSA.

Methods: Two strains of S. aureus, one MSSA (RN9120) and one MRSA (JH1) having a proclivity to gain resistance to vancomycin was used to induce resistance. The organisms were exposed to subtherapeautic VAN concentrations in a 1-compartment pharmacokinetic/pharmacodynamic (PK/PD) model, simulating human PK of VAN 200 mg q 12h over 72 h to induce resistance. At 72h, organisms recovered from the model were re-exposed to the same VAN regimen for an additional 72-144 h exposure to generate VISA. The same experiment was repeated with continuous infusion of CFZ for MSSA and bolus administration for MRSA. Changes in MIC were evaluated at the end of each 72 h exposure. A population analysis profile (PAP) was performed to evaluate for shifts in population susceptibility. All PK/PD models were completed in duplicate to ensure reproducibility.

In addition to this Time kill experiments were carried out on 10 isolates of MRSA with various phenotypes including MRSA, VISA, hVISA and LNZR strains.

Results: VAN MIC of RN 9120 and JH1 increased to 4 mg/L as soon as 144h under sub-therapeutic VAN exposure. When CFZ was concomitantly administered, VAN MIC increased to 2 mg/L at 72h. However, no further increase in MIC was noted up to 216h of sub-therapeutic VAN administration. The MIC for MRSA remained unchanged when combination of VAN and CFZ was evaluated. PAP revealed a shift in the overall population towards non-susceptibility with VAN alone. CFZ when added to VAN caused a lesser shift in MSSA and no shift in MRSA.

Time kill studies showed synergy in all 10 MRSA under study.

Conclusion: The addition of low concentration of CFZ appears to prevent emergence of VISA under sub-therapeutic exposure to VAN. The time kill study corroborates the data of advantage of using VAN in combination to CFZ. Additional studies on a wider range of isolates, more antibiotic combinations along with experiments on animal models will further validate the utility of this antibiotic combination for clinical use.