Cefotaxime Susceptibility Testing 

Cefotaxime is a third-generation cephalosporin with broad-spectrum activity against Gram-negative and Gram-positive bacteria, including Enterobacterales and Streptococcus pneumoniae. It acts by inhibiting bacterial cell wall synthesis through binding to penicillin-binding proteins (PBPs).

Testing Methodologies

The susceptibility of cefotaxime is assessed using Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) methodologies through the following approaches:

• Broth Microdilution (BMD) Method:
  • Considered the gold standard for minimum inhibitory concentration (MIC) determination.
  • Conducted in cation-adjusted Mueller-Hinton broth (CAMHB) with serial dilutions of cefotaxime.
• Etest (Gradient Strip) Method:
  • Uses a concentration gradient strip impregnated with cefotaxime.
  • Provides direct MIC determination, especially for borderline-resistant isolates.
• Automated Susceptibility Testing Systems:
  • Platforms such as VITEK 2, BD Phoenix, and Microscan determine cefotaxime MIC values.
  • Variability in detecting ESBL production requires additional confirmatory tests.

Application and Clinical Relevance

• Treatment of Severe Infections:
  • Cefotaxime is commonly used for community-acquired pneumonia (CAP), bacterial meningitis, septicemia, urinary tract infections (UTIs), and intra-abdominal infections.
• Detection and Monitoring of Resistance Mechanisms:
  • Resistance primarily arises due to ESBL production (CTX-M, TEM, SHV), AmpC β-lactamases, and carbapenemases (KPC, NDM, OXA-48, VIM, IMP).
• Surveillance and Resistance Trends:
  • Routine susceptibility testing is essential to track emerging resistance trends in hospital-acquired and community-acquired infections.
  • Helps optimize antimicrobial stewardship programs and infection control protocols.

Cefotaxime susceptibility testing is essential for clinical microbiology and infectious disease management, ensuring effective treatment of Gram-negative and Gram-positive infections. Broth microdilution and disk diffusion methods remain primary approaches, while automated systems offer high-throughput results.