Effect of beta-lactamase inhibitors on minimum inhibitory concentration of ampicillin and amoxicillin for Staphylococcus aureus strains

Abstract

Seventy strains of Staphylococcus aureus isolated from the nasopharinx (80%), urinary tract (16%), skin (1 strain) and eyes (2 strains) of patients at the clinical laboratory 'El eritrocito' were analyzed. Susceptibility to 12 antibiotics was tested by the method of Kirby-Bauer. Minimal inhibitory concentration (MIC) of ampicillin, ampicillin + sulbactam, amoxicillin and amoxicillin + clavulanic acid were determined by plate dilution. Percentages of resistance were: Penicillin and ampicillin (100%), ceftazidim (81.4%), erythromycin (68.6%), tetracyclin (31.4%) trimetoprim-sulphametoxasol (25.7%), dicloxacillin and pefloxacin (12.8%), cefuroxime and cefotaxime (4.3%), gentamicin (2.8%), cephalothin (0%). All strains were resistant to three or more antibiotics, with higher percentages of resistance to four (31.4%), three (27.1%), five (21.4%) and six (12.9%) drugs. One strain was resistant to nine antibiotics and 5.9% were resistant to seven. 97.5% of the strains were β-lactamase-positive. The MIC50 of ampicillin and amoxicillin was 500 μg/ml and the MIC90 were 1727 μg/ml and 2000 μg/ml, respectively. β-lactamase inhibitors sulbactam and clavulanic acid reduced these values eightfold, except for the MIC50 of ampicillin + sulbactam whose reduction was sixteen fold. These results show that the combination of β-lactamic + β-lactamase inhibitor was more efficient than cephalosporins for killing these β-lactamase-positive strains.