Oxacillin Susceptibility Testing

• Purpose:
  • The primary goal is to determine the susceptibility of Staphylococcus aureus strains to oxacillin, which is indicative of their susceptibility to other beta-lactam antibiotics. This testing helps in distinguishing between methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) strains.
• Methods:
  • Disk Diffusion Method: This involves placing an oxacillin-impregnated disk on an agar plate inoculated with the bacterial isolate. The zone of inhibition around the disk is measured to determine susceptibility.
  • Broth Microdilution: This method measures the minimum inhibitory concentration (MIC) of oxacillin that inhibits bacterial growth. It provides a quantitative measure of antibiotic susceptibility.
  • Molecular Methods: These include polymerase chain reaction (PCR) tests to detect the mecA gene, which is responsible for methicillin resistance. This allows for rapid detection of MRSA before culture results are available.
• Standards and Guidelines:
  • Testing procedures and interpretive criteria are standardized by organizations like the Clinical and Laboratory Standards Institute (CLSI). The CLSI document M100 outlines the methods and criteria for antimicrobial susceptibility testing, including oxacillin​ ​.
• Clinical Importance:
  • Rapid and accurate detection of oxacillin resistance is vital for infection control and treatment. In healthcare settings, early identification of MRSA can help in implementing appropriate isolation measures to prevent the spread of resistant strains​ .
• Challenges:
  • Resistance mechanisms can evolve, making it essential to update testing methods and guidelines regularly. Furthermore, accurate testing requires strict adherence to protocols to avoid errors that could lead to inappropriate antibiotic use.