Activity of clavulanate-potentiated penicillins against methicillin-resistant Staphylococcus aureus.

It has been suggested that combinations of penicillins with clavulanate may be useful in treatment of infections by methicillin-resistant strains of Staphylococcus aureus (MRSA). To determine the potentiating effect of clavulanate on the antibacterial activity of penicillins, we studied MRSA in vitro by an agar-dilution method. A total of 124 clinical isolates of MRSA were tested for sensitivity to benzylpenicillin, amoxycillin, and ticarcillin alone and in combination with 1.25, 2.5, 5.0 and 10.0 mg/l of clavulanate. Most of the strains were not typable by the international reference set of bacteriophages of human staphylococci but showed typical properties of nosocomial strains. A reduction in the MIC90 of benzylpenicillin and amoxycillin to 25 mg/l was seen in the presence of 2.5 mg/l of clavulanate. The effect was less pronounced with ticarcillin. In spite of some increase in the susceptibility of MRSA to benzylpenicillin and amoxycillin produced by clavulanate, these combinations seem to be inappropriate in infections due to MRSA.