title: Susceptibility of penicillin-susceptible and -resistant pneumococci to dirithromycin compared with susceptibilities to erythromycin, azithromycin, clarithromycin, roxithromycin, and clindamycin.
abstract: Agar dilution with incubation in air and CO2 was used to determine the MICs of erythromycin, dirithromycin, azithromycin, clarithromycin, roxithromycin, and clindamycin for 79 penicillin-susceptible, 72 penicillin-intermediate, and 74 penicillin-resistant pneumococci (158 erythromycin-susceptible and 67 erythromycin-resistant pneumococci). MICs obtained in air were usually 1 to 3 dilutions lower than those obtained in CO2. In air, the respective MICs at which 50% (MIC50s) and 90% (MIC90s) of penicillin-susceptible, -intermediate, and -resistant strains are inhibited were as follows: erythromycin, 0.016 and 0.5, 0.03 and > 64, and 2 and > 64 microg/ml; dirithromycin, 0.03 and 0.5, 0.06 and > 64, and 8 and > 64 microg/ml; azithromycin, 0.03 and 0.5, 0.06 and > 64, and 2 and > 64 microg/ml; clarithromycin, 0.016 and 0.06, 0.03 and > 64, and 2 and > 64 microg/ml; roxithromycin, 0.06 and 2, 0.06 and > 64, and 2 and > 64 microg/ml; and clindamycin, 0.03 and 0.06, 0.06 and > 64, and 0.06 and > 64 microg/ml. The MICs of erythromycin, azithromycin, and dirithromycin were very similar; however, clarithromycin MICs were generally 1 to 2 dilutions lower and roxithromycin MICs were 1 to 2 dilutions higher than those of the other compounds tested. Strains resistant to one macrolide were resistant to all macrolides; however, not all macrolide-resistant strains were resistant to clindamycin, and 32 macrolide-resistant (MICs, > or = 28 microg/ml), clindamycin-susceptible (MICs, < or = 0.25 microg/ml) strains were encountered. Time-kill testing of six strains showed similar killing kinetics for all compounds, with 99.9% killing of all strains observed with the compounds only at or above the MIC after 24 h.
authors.affiliation: Department of Pathology (Clinical Microbiology), Hershey Medical Center, Pennsylvania 17033, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Visalli
authors.forename: M R
authors.initials: MR
authors.lastname: Jacobs
authors.forename: P C
authors.initials: PC
authors.lastname: Appelbaum
mesh_terms: D000617:Aminoglycosides; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D017963:Azithromycin / Q000494:pharmacology; D017291:Clarithromycin / Q000494:pharmacology; D002981:Clindamycin / Q000494:pharmacology; D004917:Erythromycin / Q000031:analogs & derivatives / Q000494:pharmacology; D018942:Macrolides; D008826:Microbial Sensitivity Tests; D010403:Penicillin Resistance; D010406:Penicillins / Q000494:pharmacology*; D015575:Roxithromycin / Q000494:pharmacology; D013296:Streptococcus pneumoniae / Q000187:drug effects*
chemical_list: D000617:Aminoglycosides; D000900:Anti-Bacterial Agents; D018942:Macrolides; D010406:Penicillins; C053853:dirithromycin; D015575:Roxithromycin; D002981:Clindamycin; D004917:Erythromycin; D017963:Azithromycin; D017291:Clarithromycin