Fusidic acid disk diffusion testing of clostridium difficile can be calibrated using single-strain regression...

Area of Science:

• Microbiology
• Clinical Diagnostics
• Pharmacology

Background:

• Antibiotic susceptibility testing (AST) is crucial for guiding treatment decisions.
• Clostridium difficile infections require accurate susceptibility data for effective therapy.
• Standardizing disk diffusion methods ensures reliable results.

Purpose of the Study:

• To calibrate the disk diffusion antibiotic susceptibility test for fusidic acid against Clostridium difficile using single-strain regression analysis (SRA).
• To establish species-related and laboratory-specific zone breakpoints for fusidic acid susceptibility testing of C. difficile.

Main Methods:

• MIC determinations for 40 clinical C. difficile isolates using E-test.
• Standardization of disk diffusion tests according to the Swedish Reference Group for Antibiotics (SRGA).
• Calculation of SRA constants (A and B) using various fusidic acid disk concentrations (1.5-150 µg) and a control strain (ATCC 9689).

Main Results:

• SRA enabled the determination of zone breakpoints for C. difficile.
• Applying SRGA MIC limits to E-test results identified one resistant strain.
• Calculated zone breakpoints using SRA constants resulted in all strains being interpreted as susceptible.

Conclusions:

• Single-strain regression analysis (SRA) provides a method for calibrating disk diffusion tests.
• SRA facilitates the development of species-specific and laboratory-specific interpretations for antibiotic susceptibility testing.
• Calibrated disk tests improve the accuracy and reliability of fusidic acid susceptibility testing for Clostridium difficile.