[In vitro antibacterial activity of clarithromycin, a new macrolide antibiotic, and regression curve]

This study was set up to establish the regression curve for clarithromycin inhibition zone diameters (disks 15 micrograms) and MIC to create a strain distribution plot, in order to allow accurate interpretation of the disk diffusion method for testing susceptibility to clarithromycin. 430 bacterial strains were studied in three university hospital. Clarithromycin was active against erythromycin sensitive Staphylococcus aureus and coagulase negative Staphylococci at concentrations of 0.12 to 0.25 microgram/ml (mode 0.25). Erythromycin resistant strains were also resistant to clarithromycin. Enterococci could be divided into two populations, one resistant (MIC greater than 128 micrograms/ml) and the other with MIC of 0.06 to 2 (mode 0.25). This was also the case for Streptococci and Pneumococci with MIC lower for susceptible strains (mode 0.03 to 0.06). Clarithromycin was active on Haemophilus at concentrations of 4 to 64 micrograms/ml (mode 16); MICs for beta-lactamase producing strains were comparable to those of strains not producing. MICs for Neisseria were 0.12 to 16 and for B. catarrhalis 0.016 to 0.5. MIC were 0.5 and 1 (mode 1) for Clostridium perfringens; Bacteroides fragilis strains were inhibited by 0.12 to 8 micrograms/ml (mode 0.5-1). So, antibacterial activity of C was similar to that of E; it was sometimes slightly superior, particularly on Gram positive cocci. For MIC breakpoints of 1 and 4 micrograms/ml, zone size breakpoints should be 23 and 17 mm and for 2 and 8 micrograms/ml, 20 and 15 mm.