Chloramphenicol Susceptibility Testing 

Chloramphenicol is a broad-spectrum antibiotic with activity against Gram-positive and Gram-negative bacteria, including Streptococcus pneumoniae, Haemophilus influenzae, Salmonella spp., and Neisseria meningitidis. It inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, preventing peptide bond formation.

Testing Methodologies

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

• Disk Diffusion Method:
  • Uses a 30-μg chloramphenicol 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 chloramphenicol.
  • Provides direct MIC determination, particularly useful for borderline-resistant strains.
• Automated Susceptibility Testing Systems:
  • Platforms such as VITEK 2, BD Phoenix, and Microscan determine chloramphenicol MIC values.
  • May require confirmatory BMD testing for borderline-resistant isolates.

Application and Clinical Relevance

• Treatment of Serious Bacterial Infections:
  • Chloramphenicol is reserved for life-threatening infections, including bacterial meningitis, rickettsial infections, and typhoid fever.
• Detection and Monitoring of Resistance Mechanisms:
  • Resistance mechanisms include enzymatic inactivation by chloramphenicol acetyltransferase (CAT), efflux pumps (cmlA, floR genes), and target site mutations.
• Surveillance and Resistance Trends:
  • Continuous susceptibility testing is crucial for monitoring emerging resistance patterns, especially in Salmonella and Haemophilus influenzae.

Chloramphenicol susceptibility testing remains essential in diagnostic microbiology and infection control, particularly in severe bacterial infections and MDR pathogens. Broth microdilution and disk diffusion methods are the primary testing approaches, while automated systems offer high-throughput results. Given toxicity risks and emerging resistance, continuous surveillance and refinement of MIC breakpoints are necessary for ensuring optimal therapeutic decision-making.