title: Comparative in-vitro activity of azithromycin and erythromycin against Gram-positive cocci, Haemophilus influenzae and anaerobes.
abstract: The in-vitro activities of azithromycin and erythromycin were compared against 689 clinical isolates, including Gram-positive cocci, Haemophilus influenzae, and anaerobes. Of the 100 methicillin-susceptible isolates of Staphylococcus aureus tested, 77% were susceptible to 1 mg/l azithromycin and 0.5 mg/l erythromycin, whereas 22% were resistant to 32 mg/l of both compounds. All methicillin-resistant S. aureus isolates were highly resistant to both macrolides (MIC greater than 64 mg/l). Coagulase-negative staphylococci showed a wide range of susceptibilities to both compounds; MIC50 values for azithromycin and erythromycin for all isolates were 0.5 and 0.25 mg/l, respectively. With the exception of enterococci, both macrolides showed similar activity against streptococci; MIC90 values for both group A and group B streptococci were 0.03 and 0.06 mg/l for erythromycin and azithromycin, respectively. Azithromycin was less active than erythromycin against enterococci, with mode MICs of 4.0 and 1.0 mg/l, respectively; about 20% of isolates were highly resistant to both compounds. Azithromycin was substantially more active than erythromycin against H. influenzae; 41% of isolates were inhibited by 0.5 mg/l azithromycin and all isolates were inhibited by 2 mg/l. The MIC90 for erythromycin was 8 mg/l; 36% of isolates required concentrations of greater than or equal to 4 mg/l for inhibition. The anaerobic bacteria tested showed similar susceptibility to both azithromycin and erythromycin.
authors.affiliation: Department of Medical Microbiology, London Hospital Medical College, UK.
authors.forename: J P
authors.initials: JP
authors.lastname: Maskell
authors.forename: A M
authors.initials: AM
authors.lastname: Sefton
authors.forename: J D
authors.initials: JD
authors.lastname: Williams
mesh_terms: D017963:Azithromycin; D001421:Bacteria, Anaerobic / Q000187:drug effects*; D001441:Bacteroides fragilis / Q000187:drug effects; D003013:Clostridium / Q000187:drug effects; D004917:Erythromycin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D006094:Gram-Positive Bacteria / Q000187:drug effects*; D006193:Haemophilus influenzae / Q000187:drug effects*; D013210:Staphylococcus / Q000187:drug effects; D013291:Streptococcus / Q000187:drug effects
chemical_list: D004917:Erythromycin; D017963:Azithromycin