ACTIVITIES AND TIME-KILL STUDIES OF SELECTED PENICILLINS, BETA-LACTAMASE INHIBITOR COMBINATIONS, AND GLYCOPEPTIDES AGAINST ENTEROCOCCUS-FAECALIS

The activities of piperacillin, piperacillin-tazobactam, ticarcillin, ticarcillin-clavulanate, ampicillin, ampicillin-sulbactam, vancomycin, and teicoplanin were tested against 212 Enterococcus faecalis strains (9 beta-lactamase producers) by standard agar dilution MIC testing (1 0(4) CFU/spot). The MICs at which 50 and 90% of the isolates were inhi bited (MIC(50)s and MIC(90)s, respectively) were as follows (mu g/ml): piperacillin, 4 and 8; piperacillin-tazobactam, 4 and 8; ticarcillin, 64 and 128; ticarcillin-clavulanate, 64 and 128; ampicillin, 2 and 2; ampicillin-sulbactam, 1 and 2; vancomycin, 1 and 4; and teicoplanin, 0.5 and 1. Agar dilution MIC testing of the nine beta-lactamase-positi ve strains with an inoculum of 10(6) CFU/spot revealed higher beta-lac tam MICs (piperacillin, 64 to >256 mu g/ml; ticarcillin, 128 to >256 m u g/ml; and ampicillin, 16 to 128 mu g/ml); however, MICs with the add ition of inhibitors were similar to those obtained with the lower inoc ulum. Time-kill studies of 15 strains showed that piperacillin-tazobac tam was bactericidal (99.9% killing) for 14 strains after 24 h at four times the MIC, with 90% killing of all 15 strains at two times the MI C. After 12 and 6 h, 90% killing of 14 and 13 strains, respectively, w as found at two times the MIC. Ampicillin gave 99.9% killing of 14 bet a-lactamase-negative strains after 24 h at eight times the MIC, with 9 0% killing of all 15 strains at two times the MIC. After 12 and 6 h, 9 0% killing of 14 and 13 strains, respectively, was found at two times the MIC. Killing by ticarcillin-clavulanate was slower than that obser ved for piperacillin-tazobactam, relative to the MIC. For the one beta -lactamase-producing strain tested by time-kill analysis with a higher inoculum, addition of the three inhibitors (including sulbactam) to e ach of the beta-lactams resulted in bactericidal activity at 24 h at t wo times the MIC. For an enzyme-negative strain, addition of inhibitor s did not influence kinetics. Kinetics of vancomycin and teicoplanin w ere significantly slower than those of the beta-lactams, with bacteric idal activity against 6 strains after 24 h at eight times the MIC, wit h 90% killing of 12 and 14 strains, respectively, at four times the MI C. Slower-kill kinetics by both glycopeptides were observed at earlier periods.