PIPERACILLIN TAZOBACTAM COMPARED WITH CO-AMOXICLAV, AMPICILLIN PLUS SULBACTAM AND TIMENTIN AGAINST BETA-LACTAMASE-PRODUCING CLINICAL ISOLATES OF ESCHERICHIA-COLI, KLEBSIELLA-PNEUMONIAE AND KLEBSIELLA-OXYTOCA

A total of 266 enterobacterial isolates (Escherichia coli = 190, Klebsiella pneumoniae = 49, K. oxytoca = 27) were tested for susceptibility (Bauer-Kirby agar disk diffusion test and agar dilution procedure) to ampicillin, ampicillin in 10 mu g/ml of sulbactam, amoxicillin in 4 mu g/ml of clavulanic acid, piperacillin, piperacillin plus tazobactam (8:1 ratio), ticarcillin and timentin (ticarcillin in 4 mu g/ml of clavulanic acid). Discrepant results between the two methods of susceptibility testing were categorized as follows: category I = very major [minimal inhibitory concentration (MIC) = resistant, disk diffusion = susceptible] category II = major (MIC = susceptible, disk diffusion = resistant), category III = minor (MIC = intermediate susceptibility, disk diffusion = susceptible) category IV = slight (MIC = resistant, disk diffusion = intermediate), category V = minimal (MIC = susceptible, disk diffusion = intermediate) and category VI = negligible (MIC = intermediate, disk diffusion = resistant). The antibiotics, or combinations with beta-lactamase inhibitors, yielded the following discrepant results: ampicillin (II = 2, V = 1 and VI = 3), co-amoxiclav (I = 5, III = 25, IV = 1 and V = 3), ampicillin plus sulbactam (I = 5, II = 3, III = 1, V = 19 and VI = 1), piperacillin (II = 15, III = 1, V = 15 and VI = 85), piperacillin plus tazobactam (III = 16, IV = 2, V = 1 and VI = 5) and timentin (I = 2, III = 48 and IV = 1). It was proposed that the currently valid criterion (greater than or equal to 18 mm) of inhibition zones indicating susceptibility of Enterobacteriaceae to co-amoxiclav be modified to greater than or equal to 20 mm. Furthermore, a change in the inhibition zone criterion indicating intermediate susceptibility of enterobacterial isolates to piperacillin from the current 18-20 to 14-20 mm in diameter would solve the majority of encountered category VI discrepancies. It was decided to exclude ampicillin/sulbactam (10/10 mu g) and timentin (75/10 mu g) disks from the test battery for Enterobacteriaceae, but rather test isolates for susceptibility to co-amoxiclav and/or piperacillin plus tazobactam.