Antibiotic interaction of amoxycillin and clavulanic acid against 132 β-lactamase positive Haemophilus isolates: a comparison with some other oral agents

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Abstract
We studied the specific β-lactamase inhibitory activity of clavulanic acid in association with amoxycillin against 132 β-lactamase producing Haemophilus isolates. Inhibitory synergy between amoxycillin and clavulanic acid (ratio 2:1) was found in 131/132, partial synergy or antagonism in none; bactericidal synergy was found in 124/131, partial synergy in 4 and antagonism in 1. In comparison, inhibitory synergy between trimethoprim and sulphamethoxazole (ratio 1:19) was found in only 39/104 β-lactamase positive strains, partial synergy in 42 and antagonism in 3 and bactericidal synergy in 18/104, partial synergy in 8 and antagonism in 3. The amoxycillin-clavulanic acid combination expressed significantly (P < 0.001) more frequent synergy, at both inhibitory and bactericidal levels, than the trimethoprim-sulphamethoxazole combination. The synergy of amoxycillin-clavulanic acid resulted in a significant decrease of MIC90 (≥ 32.0–2.0 mg/l) and MBC90 (≥ 32.0–4.0 mg/l) of amoxycillin; the synergy of trimethoprim-sulphamethoxazole resulted in a significant decrease of MIC90 (8.0–2.0 mg/l) of trimethoprim but did not change MBC90. The amoxycillin-clavulanic acid combination was also more active than cefaclor or erythromycin alone against the 132 β-lactamase producing strains.

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