Reversion From Methicillin Susceptibility to Methicillin Resistance in Staphylococcus aureus During Treatment of Bacteremia.
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 β-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.
Department of Microbiology and Physiological Systems.
Megan K
MK
Proulx
Department of Microbiology and Physiological Systems.
Samantha G
SG
Palace
Division of Infectious Disease.
Sumanth
S
Gandra
Clinical Microbiology Laboratory, University of Massachusetts Medical School, Worcester.
Brenda
B
Torres
Section of Infectious Diseases, Department of Medicine, Boston University.
Susan
S
Weir
Massachusetts Department of Public Health, Jamaica Plain, Massachusetts.
Tracy
T
Stiles
Department of Microbiology and Physiological Systems Division of Infectious Disease.
Richard T
RT
Ellison
Department of Microbiology and Physiological Systems.
Jon D
JD
Goguen
D000368:Aged; D000595:Amino Acid Sequence; D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D016470:Bacteremia / Q000382:microbiology; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D004269:DNA, Bacterial / Q000235:genetics; D016368:Frameshift Mutation; D015964:Gene Expression Regulation, Bacterial; D006801:Humans; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000502:physiology; D009254:Nafcillin / Q000008:administration & dosage / Q000627:therapeutic use; D046915:Penicillin-Binding Proteins / Q000235:genetics / Q000378:metabolism; D016459:Prosthesis-Related Infections; D013203:Staphylococcal Infections / Q000188:drug therapy / Q000382:microbiology*; D014640:Vancomycin / Q000008:administration & dosage / Q000627:therapeutic use
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004269:DNA, Bacterial; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D009254:Nafcillin; D014640:Vancomycin
S. aureus bacteremia; fluctuation analysis; mecA PCR; oxacillin; oxacillin-susceptible mecA-positive S. aureus; β-lactam