Comparison of Clostridium difficile minimum inhibitory concentrations obtained using agar dilution vs broth...

Area of Science:

• Clinical microbiology
• Antimicrobial resistance
• Infectious diseases

Background:

• Increasing antibiotic resistance in anaerobic bacteria necessitates reliable antimicrobial susceptibility testing (AST).
• The Clinical and Laboratory Standards Institute (CLSI) recommends routine AST for Clostridium difficile.
• Accurate AST methods are vital for guiding effective treatment strategies.

Purpose of the Study:

• To compare the accuracy and reproducibility of two CLSI-recommended reference methodologies for Clostridium difficile AST.
• To evaluate the broth microdilution method against the established agar dilution method for determining minimum inhibitory concentrations (MICs).

Main Methods:

• Tested 920 Clostridium difficile isolates against seven antimicrobial agents.
• Utilized both CLSI agar dilution and broth microdilution reference methods.
• Assessed reproducibility through independent testing of a subset of isolates by two laboratories.

Main Results:

• Broth microdilution yielded MICs with a negative bias compared to agar dilution.
• Reproducibility of broth microdilution was found to be variable and dependent on the specific antimicrobial agent tested.
• Significant discrepancies were observed between the two methods.

Conclusions:

• Broth microdilution is not recommended as a reliable alternative to agar dilution for Clostridium difficile AST.
• Agar dilution remains the preferred reference method for accurate susceptibility testing of Clostridium difficile.
• Further validation is needed for alternative AST methods for anaerobic bacteria.