Antimicrobial susceptibility testing of Clostridium difficile using EUCAST epidemiological cut-off values and disk...

Area of Science:

• Clinical microbiology
• Infectious diseases
• Antimicrobial resistance

Background:

• Rising resistance of Clostridium difficile to metronidazole and vancomycin necessitates reliable antimicrobial susceptibility testing.
• The EUCAST epidemiological cut-off values (ECOFFs) provide a benchmark for interpreting susceptibility data.

Purpose of the Study:

• To evaluate disk diffusion as a method for Clostridium difficile susceptibility testing.
• To compare disk diffusion results with minimum inhibitory concentrations (MICs) from gradient tests.
• To propose zone diameter breakpoint correlates for EUCAST ECOFFs.

Main Methods:

• Tested 211 clinical Clostridium difficile isolates from Danish hospitals and 10 isolates with known reduced susceptibility.
• Utilized Etest gradient strips and disk diffusion assays for metronidazole, vancomycin, and moxifloxacin.
• Grew isolates on supplemented Brucella Blood Agar.

Main Results:

• Demonstrated excellent agreement between inhibition zone diameters and MICs.
• Observed minimal variation in zone diameters for each MIC value (93% within 6 mm for metronidazole, 95% within 4 mm for vancomycin, 98% within 4 mm for moxifloxacin).
• Proposed zone diameter breakpoints (Metronidazole ≥ 23 mm, Vancomycin ≥ 19 mm, Moxifloxacin ≥ 20 mm) resulted in <2% major errors and no very major errors.

Conclusions:

• Disk diffusion is a viable and accurate method for antimicrobial susceptibility testing of Clostridium difficile.
• The proposed zone diameter breakpoints can be used to interpret disk diffusion results against EUCAST ECOFFs.
• This method supports clinical decision-making in managing Clostridium difficile infections.