Antibacterial activity and synergy, in vitro and in vivo, of a combination of amoxycillin and flucloxacillin.

The antibacterial activity of a combination of equal parts of amoxycillin and flucloxacillin was compared in vitro and in vivo with that of amoxycillin and flucloxacillin against a range of gram-positive and gram-negative bacteria. The combination generally showed additive effects against bacteria sensitive to the individual penicillins and there was no evidence of antagonism, but synergistic effects were observed between amoxycillin and flucloxacillin against certain amoxycillin-resistant gram-negative bacilli. The extent of synergism varied according to the particular bacterial species under test and synergy was observed only against bacteria with chromosomally-mediated beta-lactamases and not against bacteria with R-factor-mediated beta-lactamases. In general, amoxycillin + flucloxacillin demonstrated activity against experimental mouse infections in good agreement with demonstrated activity against experimental mouse infections in good agreement with its in vitro activity, and synergy was produced against a range of gram-negative bacilli in vivo. The data suggest that clinical trial with amoxycillin + flucloxacillin in the treatment of selected infections including those due to some amoxycillin-resistant bacteria may well be justified.