SUSCEPTIBILITY TESTING OF MACROLIDE AND LINCOSAMIDE ANTIBIOTICS ACCORDING TO DIN GUIDELINES

The in-vitro activity of azithromycin, clarithromycin, erythromycin, j osamycin, midekamycin, roxithromycin and clindamycin against 674 Gram- negative and Gram-positive clinical isolates, including methicillin-re sistant Staphylococcus aureus, was determined by agar dilution, microd ilution and agar diffusion with Mueller-Hinton medium according to the Deutsches Institut fur Normung (DIN) 58940 guidelines. The results ob tained by regression analysis and the error-rate-bounded method of Met zler-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 value s are suggested for 15 mu g macrolide discs: for susceptible Gram-posi tive and Gram-negative strains, an inhibition zone diameter (IZD) of g reater than or equal to 26 mm at a corresponding MIC of less than or e qual to 2 mg/L; for resistant Gram-positive strains, an IZD of less th an or equal to 21 mm; for resistant Gram-negative strains, an IZD of l ess than or equal to 19 mm at a corresponding MIC of greater than or e qual to 8 mg/L. For Haemophilus influenzae only, breakpoints for azith romycin are suggested with IZDs of greater than or equal to 21 mm for susceptible and less than or equal to 18 mm for resistant.