Susceptibility testing of macrolide and lincosamide antibiotics according to DIN guidelines. Deutsches Institut fur Normung

The in-vitro activity of azithromycin, clarithromycin, erythromycin, josamycin, midekamycin, roxithromycin and clindamycin against 674 Gram-negative and Gram-positive clinical isolates, including methicillin-resistant Staphylococcus aureus, was determined by agar dilution, microdilution and agar diffusion with Mueller-Hinton medium according to the Deutsches Institut für Normung (DIN) 58940 guidelines. The results obtained by regression analysis and the error-rate-bounded method of Metzler-DeHaan indicate that common interpretative criteria (breakpoints) for test discs may be assigned to susceptible/resistant Gram-positive strains for all antibiotics tested. The following tentative DIN values are suggested for 15 microg macrolide discs: for susceptible Gram-positive and Gram-negative strains, an inhibition zone diameter (IZD) of > or = 26 mm at a corresponding MIC of < or = 2 mg/L; for resistant Gram-positive strains, an IZD of < or = 21 mm; for resistant Gram-negative strains, an IZD of < or = 19 mm at a corresponding MIC of > or = 8 mg/L. For Haemophilus influenzae only, breakpoints for azithromycin are suggested with IZDs of > or = 21 mm for susceptible and < or = 18 mm for resistant.