In-vitro bactericidal activity of four oral antibiotics against pathogens responsible for acute otitis media in children.

This study was designed to test the in-vitro activity of four oral antibiotics against the four microorganisms most frequently isolated in acute otitis media: beta-lactamase-positive Haemophilus influenzae (N = 10), beta-lactamase-positive Moraxella catarrhalis (N = 10), penicillin-sensitive Streptococcus pneumoniae (N = 11) and methicillin-sensitive Staphylococcus aureus (N = 10), by the bactericidal curve method. Bactericidal kinetics were determined for concentrations of antibiotic equivalent to those found in the middle ear after treatment: amoxycillin-clavulanic acid (2.5 mg l-1/0.6 mg l-1 and 2.5 mg l-1/1.2 mg l-1), cefaclor (1 mg l-1), erythromycin (0.5 mg l-1) and erythromycin/sulfisoxazole (0.2/3 mg l-1). The inoculum was of 10(6) colony-forming units (cfu) ml-1. The bacterial counts were performed after 5 h and 24 h using a spiral inoculator system. The results showed that amoxycillin-clavulanic acid had rapid bactericidal activity (< 24 h) on the tested organisms at each of the doses used (reduction < or = 3 log10 cfu ml-1) which was not observed with the other antibiotics at either 5 or 24 h. Erythromycin alone or combined with sulfisoxazole had a bacteriostatic effect on Moraxella catarrhalis and Streptococcus pneumoniae but not on Haemophilus influenzae or Staphylococcus aureus. Cefaclor had no bactericidal action under these conditions.