In vitro activity of amoxicillin plus clavulanic acid against Haemophilus influenzae and Branhamella catarrhalis.

The in vitro activity of amoxicillin in the presence of clavulanic acid against clinical isolates of Haemophilus influenzae and Branhamella catarrhalis was assessed in comparison with ampicillin, amoxicillin, cefaclor and erythromycin. The isolates were selected so as to yield equal numbers of beta-lactamase producing and non-beta-lactamase producing strains of the two species. MICs obtained by agar dilution indicated that amoxicillin in the presence of clavulanic acid was the most active of the drugs tested. Clavulanic acid potentiated the activity of amoxicillin against beta-lactamase-producing strains of both Haemophilus influenzae and Branhamella catarrhalis. Further studies on a few strains of each species revealed that the beta-lactamase of Haemophilus influenzae (TEM-1) rapidly inactivated ampicillin and slowly inactivated cefaclor but not cefuroxime. The Branhamella catarrhalis enzyme rapidly inactivated cefaclor, ampicillin and to some extent cefuroxime. Clavulanic acid afforded protection against the beta-lactamase action of both species when beta-lactam antibiotics were added to bacterial cultures.