G Kronvall1. 1. Clinical Microbiology-MTC, Karolinska Institute, Karolinska Hospitals, Stockholm, Sweden. goran.kronvall@ki.se
Abstract
OBJECTIVE: To investigate the possibility of estimating the MICs of fusidic acid and ciprofloxacin for bacterial isolates using series of antibiotic disk concentrations in diffusion tests, so-called M-tests. METHODS: Thirty Staphylococcus aureus and S. epidermidis strains were tested for fusidic acid susceptibility. Sixty-one clinical isolates of eight bacterial species were tested for ciprofloxacin susceptibility. Disk diffusion was standardized according to the Swedish reference group for antibiotics (SRGA). For fusidic acid, a series of disks (1.5, 5.0, 15, 50 and 150 microg) was used. Ciprofloxacin was applied in four different diffusion sources (1, 3, 10 and 30 microg) on a single strip, the M-strip, and used. True MIC values were determined using the standardized agar dilution method according to the SRGA. Single-strain regression analysis (SRA) was employed to calculate critical concentration equivalents (Qzero). RESULTS: Fusidic acid and ciprofloxacin critical concentrations were determined for the bacterial isolates. The mean conversion factors for Qzero to yield the true MIC were 2.06 (range 0.34-8.9) for fusidic acid and 2.05 (range 0.37-8.1) for ciprofloxacin. There was a correlation between true MIC values (all MICs expressed as 2 log + 9) and the calculated MIC values (Qzero x conversion factor) for both fusidic acid (R = 0.9822) and ciprofloxacin (R = 0.9696). CONCLUSIONS: MIC values of clinical isolates can be estimated using SRA calculations on zone measurements in disk tests with several concentrations of the antibiotic in diffusion sources.
OBJECTIVE: To investigate the possibility of estimating the MICs of fusidic acid and ciprofloxacin for bacterial isolates using series of antibiotic disk concentrations in diffusion tests, so-called M-tests. METHODS: Thirty Staphylococcus aureus and S. epidermidis strains were tested for fusidic acid susceptibility. Sixty-one clinical isolates of eight bacterial species were tested for ciprofloxacin susceptibility. Disk diffusion was standardized according to the Swedish reference group for antibiotics (SRGA). For fusidic acid, a series of disks (1.5, 5.0, 15, 50 and 150 microg) was used. Ciprofloxacin was applied in four different diffusion sources (1, 3, 10 and 30 microg) on a single strip, the M-strip, and used. True MIC values were determined using the standardized agar dilution method according to the SRGA. Single-strain regression analysis (SRA) was employed to calculate critical concentration equivalents (Qzero). RESULTS:Fusidic acid and ciprofloxacin critical concentrations were determined for the bacterial isolates. The mean conversion factors for Qzero to yield the true MIC were 2.06 (range 0.34-8.9) for fusidic acid and 2.05 (range 0.37-8.1) for ciprofloxacin. There was a correlation between true MIC values (all MICs expressed as 2 log + 9) and the calculated MIC values (Qzero x conversion factor) for both fusidic acid (R = 0.9822) and ciprofloxacin (R = 0.9696). CONCLUSIONS: MIC values of clinical isolates can be estimated using SRA calculations on zone measurements in disk tests with several concentrations of the antibiotic in diffusion sources.