Journal
JOURNAL OF INFECTIOUS DISEASES
Volume 213, Issue 6, Pages 1041-1048Publisher
OXFORD UNIV PRESS INC
DOI: 10.1093/infdis/jiv512
Keywords
oxacillin-susceptible mecA-positive S. aureus; fluctuation analysis; mecA PCR; S. aureus bacteremia; beta-lactam; oxacillin
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Funding
- University of Massachusetts Medical School
- National Institutes of Health (NIH) [T32 AI095213]
- National Institute of Allergy and Infectious Diseases, NIH [HHSN272200700055C]
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Approximately 3% of Staphylococcus aureus strains that, according to results of conventional phenotypic methods, are highly susceptible to methicillin-like antibiotics also have polymerase chain reaction (PCR) results positive for mecA. The genetic nature of these mecA-positive methicillin-susceptible S. aureus (MSSA) strains has not been investigated. We report the first clearly defined case of reversion from methicillin susceptibility to methicillin resistance among mecA-positive MSSA within a patient during antibiotic therapy. We describe the mechanism of reversion for this strain and for a second clinical isolate that reverts at a similar frequency. The rates of reversion are of the same order of magnitude as spontaneous resistance to drugs like rifampicin. When mecA is detected by PCR in the clinical laboratory, current guidelines recommend that these strains be reported as resistant. Because combination therapy using both a beta-lactam and a second antibiotic suppressing the small revertant population may be superior to alternatives such as vancomycin, the benefits of distinguishing between mecA-positive MSSA and MRSA in clinical reports should be evaluated.
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