Post Traumatic Wound Infection: Testing for Methicillin Resistance

MRSA Disk Test

  • The disk diffusion test can also be used in the detection of methicillin-resistant Staphylococcus aureus.
  • Illustrated in the photograph is the surface of a Mueller-Hinton agar plate previously inoculated with a strain of S. aureus suspected of being methicillin-resistant. Although the zone of inhibition is at the borderline for resistance (18 mm); the presence of small colonies within the zone of inhibition (yellow arrows) indicates the presence of heteroresistant strains.
  • The interpretation here, therefore, is "methicillin-resistant" staphylococci, even though the zone diameter appears to be adequate. The detection of the heteroresistant strains indicates that MIC studies are required.

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