COMPARATIVE ACTIVITIES OF CLARITHROMYCIN, ERYTHROMYCIN, AND AZITHROMYCIN AGAINST PENICILLIN-SUSCEPTIBLE AND PENICILLIN-RESISTANT PNEUMOCOCCI

Activities of clarithromycin, erythromycin, and azithromycin against 1 20 pneumococci from the United States were tested by agar dilution MIC . All three compounds yielded MICs at which 90% of the isolates were i nhibited (MIC(90)s) of less than or equal to 0.125 mu g/ml against pen icillin-susceptible and -intermediate strains, but MIC(90)s against re sistant strains were > 128.8 mu g/ml. All erythromycin-resistant strai ns were also resistant to clarithromycin and azithromycin. Clarithromy cin yielded MICs which were generally one or two dilutions lower than those of the other two compounds for all strains. The respective bacte riostatic and bactericidal values (micrograms per milliliter) for two susceptible, two intermediate, and two resistant strains were 0.004 to 0.03 and 0.016 to 0.03 (0.004 to 0.03/0.016 to 0.03) (clarithromycin) , 0.008 to 0.06/0.016 to 0.125 (erythromycin), and 0.016 to 0.06/0.03 to 0.125 (azithromycin); clarithromycin yielded the lowest values, All compounds were uniformly bactericidal after 24 h only; erythromycin w as bactericidal at eight times the MIC, and azithromycin and clarithro mycin were both bactericidal at two times the MIC. The relevance of th ese in vitro differences requires clarification by clinical trials.