MIC determination of fusidic acid and of ciprofloxacin using multidisk diffusion tests.
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.
Kronvall
G
G
Clinical Microbiology-MTC, Karolinska Institute, Karolinska Hospitals, Stockholm, Sweden. goran.kronvall@ki.se
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D000890:Anti-Infective Agents / Q000494:pharmacology*; D002939:Ciprofloxacin / Q000494:pharmacology*; D005672:Fusidic Acid / Q000494:pharmacology*; D006090:Gram-Negative Bacteria / Q000187:drug effects; D006094:Gram-Positive Bacteria / Q000187:drug effects; D005779:Immunodiffusion; D008826:Microbial Sensitivity Tests; D018410:Pneumonia, Bacterial / Q000097:blood / Q000382:microbiology; D012044:Regression Analysis; D013211:Staphylococcus aureus / Q000187:drug effects*; D013212:Staphylococcus epidermidis / Q000187:drug effects*; D014376:Tuberculosis / Q000097:blood / Q000382:microbiology
D000900:Anti-Bacterial Agents; D000890:Anti-Infective Agents; D005672:Fusidic Acid; D002939:Ciprofloxacin