The Clinical Relevance of Heterogeneous Beta-lactam Resistance in Ca-usa300
Open Access
- Author:
- Inglis, George
- Area of Honors:
- Veterinary and Biomedical Sciences
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Michael Mugo Mwangi, Thesis Supervisor
Dr. Lester C Griel Jr., Thesis Honors Advisor - Keywords:
- S. aureus
MRSA
USA300
HBLR - Abstract:
- In order for a strain of Staphylococcus aureus to be classified as MRSA, it must possess the gene mecA and its product, PBP2a. However, while mecA is essential for high levels of resistance to β-lactam antibiotics, it is not sufficient. In fact, previous studies have demonstrated that clinical isolates of community-associated USA300 can possess a wide variety of MIC99 values between cells, ranging from 0.1-1000 μg/mL OX. However, only 0.1% of these cells account for resistance above 10 μg/mL OX range. This phenomenon of heterogeneous β-lactam resistance (HBLR), where a vast majority (99.9%) of cells in the same strain are weakly resistant and a small subpopulation are highly resistant, may explain why clinical treatments of CA-MRSA strains such as USA300 with β-lactams are typically ineffective. While HBLR has been demonstrated to occur through various in vitro experiments, very little work has been done to show that it can occur in vivo and thus is clinically relevant. In this study, we subjected a simulated endocardial vegetation of USA300 to a 72-hour oxacillin treatment to replicate the in vivo treatment of endocarditis in a patient, hypothesizing that the final vegetation would primarily consist of bacterial cells with greater β-lactam resistance than the wild-type or initial inoculum. Colonies isolated from the vegetation after this 72 hour period were characterized in contrast to wild-type USA300. Multiple analyses on the resistance and fitness levels of these samples confirmed that the colonies present after the in vivo simulation were significantly more resistant to β-lactams than wild-type USA300. In addition, genomic analyses revealed these isolates were part of a unique subpopulation of spontaneous mutants, ultimately supporting that HBLR is a clinically relevant phenomenon.