Effective in Vitro Synergy of Piperacillin/Tazobactam Plus either Netilmicin or Aztreonam against Metallo-β-lactamase-producing Pseudomonas Aeruginosa

This study was designed to compare the activity of piperacillin/tazobactam (TZP) in combination with either netilmicin (NET) or aztreonam (ATM) against metallo-β lactamases (MβL) Pseudomonas aeruginosa. Out of 46 nosocomial Ps. aeruginosa isolates, 38 (82.6%) isolates were carbapenem-resistant by disc diffusion susceptibility testing. These were then screened by imipenem + EDTA combined disc test for MβL production. Thirty (78.9%) MβL-producing isolates detected were further tested for antibiotic synergy against NET-TZP and ATM-TZP combinations by Etest synergy and Etest/agar dilution tests. Neither combination showed antagonism. The synergistic effect of NET-TZP combination was detected in [27 (90%) and 28 (93.3%), respectively] and ATM-TZP combination in [24 (80%) and 25 (83.3%), respectively] of the tested isolates by Etest synergy and Etest/agar dilution tests, respectively. Thus, we have determined the synergistic effect of these two combinations for treating MβL-producing Ps. aeruginosa. Significance and Impact of the Study: The present study has shown valuable information on the combination treatment of Ps. aeruginosa and recommends their use for treatment of resistant Ps. aeruginosa.