In vitro synergy of ticarcillin/clavulanate in combination with aztreonam and ceftolozane/tazobactam against SPM-1-producing Pseudomonas aeruginosa strains

ABSTRACT Minimal inhibitory concentrations (MICs) of ticarcillin/clavulanic acid (TLc), ceftolozane/tazobactam (C/T), and aztreonam (AT) were determined for six SPM-1-producing Pseudomonas aeruginosa (PSA) using Etest® strips and the synergistic effect of such antimicrobials against was evaluated by gradient diffusion strip crossing (GDSC) test. The fraction inhibitory concentration indexes (FICI) were calculated and showed a synergistic (n=3) and additive (n=2) effects of TLc+AT against SPM-1 producers, while TLc+C/T combination caused no effect. Average MIC reduction of TLc and AT by GDSC was 3-fold and 2-fold dilutions, respectively. Thus, TLc+AT might be a candidate as a combination therapy to treat SPM-1-producing PSA infections.