Effect of beta-lactamase inhibitors on minimum inhibitory concentration of ampicillin and amoxicillin for Staphylococcus aureus strains.
Seventy strains of Staphylococcus aureus isolated from the nasopharynx (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%), ceftazidime (81.4%), erythromycin (68.6%), tetracycline (31.4%) trimethoprim-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 beta-lactamase-positive. The MIC50 of ampicillin and amoxicillin was 500 micrograms/ml and the MIC90 were 1727 micrograms/ml and 2000 micrograms/ml, respectively. beta-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 beta-lactamic + beta-lactamase inhibitor was more efficient than cephalosporins for killing these beta-lactamase-positive strains.
Escuela Nacional de Estudios Profesionales Iztacala, UNAM, México.
G L
GL
Paniagua
E
E
Monroy
O
O
García
S
S
Vaca
D042462:4-Quinolones; D000617:Aminoglycosides; D000658:Amoxicillin / Q000494:pharmacology*; D000667:Ampicillin / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D000890:Anti-Infective Agents / Q000494:pharmacology; D001426:Bacterial Proteins / Q000037:antagonists & inhibitors*; D002511:Cephalosporins / Q000494:pharmacology; D019818:Clavulanic Acid / Q000494:pharmacology*; D004352:Drug Resistance, Microbial; D018432:Drug Resistance, Multiple; D004359:Drug Therapy, Combination / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D018942:Macrolides; D008826:Microbial Sensitivity Tests; D010406:Penicillins / Q000494:pharmacology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000201:enzymology; D013407:Sulbactam / Q000494:pharmacology*; D065093:beta-Lactamase Inhibitors
D042462:4-Quinolones; D000617:Aminoglycosides; D000900:Anti-Bacterial Agents; D000890:Anti-Infective Agents; D001426:Bacterial Proteins; D002511:Cephalosporins; D004791:Enzyme Inhibitors; D018942:Macrolides; D010406:Penicillins; D065093:beta-Lactamase Inhibitors; D019818:Clavulanic Acid; D000667:Ampicillin; D000658:Amoxicillin; D013407:Sulbactam