Susceptibility Testing 

Cefepime is a fourth-generation cephalosporin with extended-spectrum activity against Gram-positive and Gram-negative pathogens, including Pseudomonas aeruginosa and Enterobacterales. Its mechanism of action involves inhibition of bacterial cell wall synthesis through high-affinity binding to penicillin-binding proteins (PBPs), leading to bacterial lysis. 

Testing Methodologies

The susceptibility of cefepime is evaluated using standardized Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines through the following methodologies:

• Broth Microdilution (BMD) Method:
  • Gold standard for determining minimum inhibitory concentration (MIC) values.
  • Conducted in cation-adjusted Mueller-Hinton broth (CAMHB) with serial dilutions of cefepime.
  • CLSI-defined MIC breakpoints for Enterobacterales and Pseudomonas aeruginosa:
    • Susceptible (S): ≤2 μg/mL
    • Intermediate (I): 4-8 μg/mL
    • Resistant (R): ≥16 μg/mL
• Etest (Gradient Strip) Method:
  • Employs a concentration gradient strip impregnated with cefepime.
  • Provides direct MIC determination, beneficial for testing borderline-resistant isolates.

Application and Clinical Relevance

• Treatment of Severe Gram-Negative Infections:
  • Cefepime is a first-line agent for hospital-acquired pneumonia (HAP), febrile neutropenia, urinary tract infections (UTIs), intra-abdominal infections, and bloodstream infections.
  • Effective against Pseudomonas aeruginosa, but combination therapy is preferred in critically ill patients.
• Detection and Monitoring of Resistance Mechanisms:
  • Resistance mechanisms include ESBLs (CTX-M, TEM, SHV), AmpC β-lactamases, and carbapenemases (KPC, NDM, OXA-48, VIM, IMP).
  • AmpC-overproducing organisms may exhibit inducible resistance, necessitating confirmatory cefoxitin screening and inhibitor-based testing.
• Surveillance and Resistance Trends:
  • Continuous susceptibility testing is crucial to track emerging resistance patterns.
  • Helps guide antimicrobial stewardship programs and infection control policies.

Cefepime susceptibility testing remains integral to clinical microbiology, ensuring effective treatment of multidrug-resistant (MDR) Gram-negative infections. The broth microdilution and disk diffusion methods are primary approaches, while automated testing systems provide high-throughput analysis.