Reversion From Methicillin Susceptibility to Methicillin Resistance in Staphylococcus aureus During Treatment of Bacteremia

UMMS Affiliation

Department of Medicine, Division of Infectious Diseases and Immunology; Department of Microbiology and Physiological Systems; Clinical Microbiology Laboratory

Publication Date


Document Type



Immunology of Infectious Disease | Infectious Disease


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.

DOI of Published Version



J Infect Dis. 2015 Oct 26. pii: jiv512. Link to article on publisher's site

Journal/Book/Conference Title

The Journal of infectious diseases

Related Resources

Link to Article in PubMed

PubMed ID