Characterization of oxacillin-susceptible mecA-positive Staphylococcus aureus: A new type of MRSA

Yasuko Hososaka, Hideaki Hanaki, Harumi Endo, Yumiko Suzuki, Zenzo Nagasawa, Yoshihito Otsuka, Taiji Nakae, Keisuke Sunakawa

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99 Citations (Scopus)


Methicillin-resistant Staphylococcus aureus (MRSA) has been defined as S. aureus having the mecA gene or showing a minimum inhibitory concentration (MIC) of oxacillin higher than 4 mg/l. However, some clinical isolates are mecA-positive and oxacillin-susceptible. Therefore, we surveyed the occurrence of S. aureus having the mecA gene and an MIC of oxacillin of less than 2 mg/l (oxacillin-susceptible MRSA; OS-MRSA) in a total of 480 strains of S. aureus collected from 11 hospitals in different location in Japan isolated from 2003 through 2005. We found 6 strains matching the criteria for OS-MRSA. All 6 strains were staphylococcal cassette chromosome (SCC) mec-positive, without exception, and 4 strains showed the SCCmec type III-variant, which is unique in Japan. These OS-MRSAs were least resistant to oxacillin among the MRSAs tested and they were within the susceptible range to seven other β-lactam antibiotics tested. Thus, OS-MRSA may become a high-resistant MRSA upon the treatment of patients with β-lactam antibiotics. To characterize whether these OS-MRSAs were hospital-acquired or community-acquired MRSAs, we tested for the presence of the genes encoding toxins. Genes encoding hemolysin, exfoliative toxin, enterotoxin, toxic shock syndrome toxin-1, and Panton-Valentine leukocidin were found in 6, 4, 0, 0, and 0 strains, respectively. These results revealed that OS-MRSAs could be classified as a new type of MRSA that exhibits properties distinguishable from either hospital- or community-acquired MRSA. Coagulase typing of the OS-MRSAs supported the above conclusion. In this study, the occurrence of OS-MRSA at a certain frequency was noted; precautions are called for in the classification of oxacillin-resistant S. aureus and in the treatment of OS-MRSA infection.

Original languageEnglish
Pages (from-to)79-86
Number of pages8
JournalJournal of Infection and Chemotherapy
Issue number2
Publication statusPublished - 2007 Apr
Externally publishedYes


  • Community-acquired MRSA (CA-MRSA)
  • Hospital-acquired MRSA (HA-MRSA)
  • Oxacillin-susceptible methicillin-resistant S. aureus (OS-MRSA)
  • Staphylococcus cassette chromosomal mecA (SCCmec)
  • Toxins

ASJC Scopus subject areas

  • Microbiology (medical)
  • Infectious Diseases
  • Pharmacology (medical)


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