Fusidic acid disk diffusion testing of Clostridium difficile can be calibrated using single-strain regression analysis

Single-strain regression analysis (SRA) was employed to calibrate the disk diffusion antibiotic susceptibility test for fusidic acid and Clostridium d ifficile. MIC determinations of 40 clinical isolates of C. difficile were p erformed,with the E-test. The disk diffusion test Iras standardized accordi ng to the Swedish Reference Group for Antibiotics (SRGA). Disks used for SR A contained 1.5, 5, 15, 50 and 150 mug fusidic acid and the routine disk co ntained 50 mug fusidic acid. A control strain, ATCC 9689, was also tested. SRA constants A and B of the regression lines were calculated. This permitt ed the determination of zone breakpoints for C. difficile. When applying th e pharmacological MIC S and R limits set by SRGA to the E-test results 1 st rain of C. difficile was interpreted as resistant. Zone breakpoints corresp onding to the pharmacological MIC limits and calculated using the mean SRA constants for the 40 clinical isolates lead to all strains being interprete d as susceptible. SRA calculations enable laboratories to set up calibrated disk tests with species-related and laboratory-specific interpretations.