In vitro activity of roxithromycin against hospital bacteria and the concordance curve

: This study was set up to establish the regression curve for roxithromycin 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 roxithromycin. 373 bacterial strains were studied in three university hospital. Roxithromycin was active against erythromycin sensitive Staphylococcus aureus and coagulase negative Staphylococci at concentrations of 0.06 to 4 micrograms/ml (mode 0.5). Erythromycin resistant strains were also resistant to roxithromycin. Enterococci could be divided into two populations, one resistant (MIC greater than 128 micrograms/ml) and the other with MIC of 0.5 to 32 (mode 1-2). This was also the case for Streptococci and Pneumococci with MIC lower for susceptible strains (mode 0.06-0.12). Roxithromycin was active on Haemophilus at concentrations of 0.12 to 32 micrograms/ml; MIC for beta-lactamase producing strains were comparable to those of strains not producing. MIC for Gonococci were low (less than 0.008 to 0.12), except for three strains. They were higher for Meningococci (0.03 to 32) with a majority of strains inhibited by 0.5 to 4 micrograms/ml. MIC were 4 for Clostridium perfringens; Bacteroides fragilis strains were inhibited by 0.5 to 2 micrograms/ml. The correlation coefficient for regression curve was 0.79; for critical concentrations less than or equal to 1 and greater than 4 micrograms/ml, critical diameters are greater than or equal to 22 and less than 17 mm.