Cefoxitin Susceptibility Testing 

Cefoxitin is a second-generation cephalosporin classified as a cephamycin. It exhibits broad-spectrum activity against Gram-negative bacteria, some anaerobes, and limited activity against Gram-positive bacteria. Cefoxitin is commonly used for surgical prophylaxis and the treatment of intra-abdominal infections. Additionally, it is widely used in the detection of methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci (CoNS) because cefoxitin serves as a surrogate marker for mecA gene-mediated resistance.

Content:

• Testing Methods:
  • Disk Diffusion (Kirby-Bauer Method):
    • Disks containing cefoxitin (30 µg) are placed on Mueller-Hinton agar plates inoculated with the bacterial isolate.
    • After incubation, the diameter of the inhibition zone is measured and interpreted using CLSI or EUCAST guidelines.
  • Minimum Inhibitory Concentration (MIC) Testing:
    • MIC testing is performed using broth microdilution, agar dilution, or automated systems.
    • The MIC is the lowest concentration of cefoxitin that inhibits visible bacterial growth.
  • E-test (Gradient Diffusion):
    • E-test is not commonly used for cefoxitin susceptibility testing but may be employed in specific situations.

Application:

• Clinical Use:
  • Cefoxitin is used for:
    • Surgical prophylaxis, particularly in abdominal and colorectal surgeries.
    • Intra-abdominal infections, often combined with other agents for anaerobic coverage.
    • Gynecological infections, including pelvic inflammatory disease (PID).
    • Skin and soft tissue infections caused by susceptible organisms.
  • MRSA Detection:
    • Cefoxitin susceptibility testing is widely used to detect methicillin resistance in Staphylococcus aureus and CoNS as it provides a more reliable surrogate than oxacillin.
• Antimicrobial Stewardship:
  • Promotes the use of cefoxitin in targeted therapy, reducing reliance on carbapenems or other broad-spectrum agents.
  • Supports switching to narrower-spectrum antibiotics when susceptibility results are available.

Cefoxitin susceptibility testing is critical for managing infections caused by Gram-negative bacteria, Gram-positive bacteria (particularly Staphylococcus aureus), and anaerobes. It is also a gold standard surrogate for detecting methicillin resistance in Staphylococcus species. Standardized methods like disk diffusion and MIC testing provide reliable results.