Ceftazidime  Susceptibility Testing 

Ceftazidime is a third-generation cephalosporin with broad-spectrum activity against Gram-negative bacteria, including Pseudomonas aeruginosa. It is primarily used in treating nosocomial infections, febrile neutropenia, and multidrug-resistant (MDR) Gram-negative infections. While highly effective against Enterobacterales and non-fermenters, its susceptibility to extended-spectrum β-lactamases (ESBLs) and carbapenemases necessitates routine susceptibility testing.

Testing Methodologies

The susceptibility of ceftazidime is determined using Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines, employing the following methodologies:

• Broth Microdilution (BMD) Method:
  • Considered the gold standard for minimum inhibitory concentration (MIC) determination.
  • Conducted in cation-adjusted Mueller-Hinton broth (CAMHB).
• Disk Diffusion Method:
  • Uses a 30-μg ceftazidime disk on Mueller-Hinton agar (MHA).
  • Incubation at 35°C for 16-18 hours.
• Etest (Gradient Strip) Method:
  • Utilizes a concentration gradient strip impregnated with ceftazidime.
  • Provides direct MIC determination, particularly useful for MDR strains requiring combination therapy.

Application and Clinical Relevance

• Treatment of Multidrug-Resistant Gram-Negative Infections:
  • Ceftazidime is a key antibiotic for treating Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterobacter spp..
  • Often used in combination with β-lactamase inhibitors (e.g., avibactam, tazobactam) for ESBL- and AmpC-producing strains.
• Detection and Monitoring of Resistance Mechanisms:
  • Resistance mechanisms include ESBL production (CTX-M, TEM, SHV), AmpC β-lactamases, and carbapenemases (KPC, NDM, OXA-48, VIM, IMP).
  • Confirmatory ESBL phenotypic testing (e.g., double-disk synergy test or combination disk tests) is essential.
• Surveillance and Resistance Trends:
  • Continuous susceptibility testing is crucial for monitoring emerging resistance patterns.
  • Helps optimize antimicrobial stewardship programs by preventing empirical overuse.

Ceftazidime susceptibility testing plays a critical role in diagnostic microbiology and infection control, particularly in nosocomial and MDR infections. Broth microdilution and disk diffusion methods remain primary approaches, while automated systems offer rapid assessment. Due to the increasing prevalence of β-lactamase-producing pathogens, ongoing surveillance and refinement of MIC breakpoints are essential for optimal therapeutic decision-making.