Mechanisms of methicillin resistance in staphylcoccus aureus and methods for laboratory detection

James H. Jorgensen

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35 Scopus citations


Three distinctly different mechanisms of methicillin resistance have been described in Staphylococcus aureus. The best-documented and probably most important mechanism is production of a unique, low affinity penicillin-binding protein, PBP 2aStrains possessing PBP 2aare resistant to methicillin, oxacillin, and probably all other currently available b-lactam antibiotics. Two additional mechanisms of reduced susceptibility to methicillin have been described. Borderline resistance (BORSA) to the semi-synthetic penicillins has been attributed to the hyperproduction of normal staphylococcal b-lactamase. A third mechanism has recently been advanced that describes an intermediate level of resistance to methicillin due to production of modified, normal PBPs with reduced affinity for b-lactams (MODSA). Little is known regarding the prevalence or clinical significance of the BORSA and MODSA strains. The most reliable in vitro susceptibility test methods for detecting MRSA (strains possessing PBP 2,) include the microdilution minimum inhibitory concentration (MIC) test (with 2% NaCl supplemented broth), the oxacillin agar screen plate test (incorporating 6 ug/ml oxacillin in 4% NaCl supplemented agar), and the National Committee for Clinical LaboratoryStandards (NCCLS) disk diffusion test with oxacillin. All three methods use direct inoculumpreparation and incubation of tests at 35°C for a full 24 hours. (Infect Control Hosp Epidemiol. 1991;12:14-19.).

Original languageEnglish (US)
Pages (from-to)14-19
Number of pages6
JournalInfection Control & Hospital Epidemiology
Issue number1
StatePublished - Jan 1991

ASJC Scopus subject areas

  • Epidemiology
  • Microbiology (medical)
  • Infectious Diseases


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