Synergistic effect of polyethylenimine plus piperacillin/tazobactam on clinicalPseudomonas aeruginosa isolates resistant to piperacillin/tazobactam

Abstract

We investigated the in vitro susceptibility of piperacillin/tazobactam in association with polyethylenimine (PEI), a synthetic polycation polymer, against 24 clinical isolates of Psedomonas aeruginosa resistant to piperacillin/tazobactam. The strains were isolated from patients with respiratory tract infections. MIC of piperacillin/tazobactam was determined by agar dilution method in accordance with NCCLS methodology. An inoculum of 10 4 CFU/spot with or without PEI (250 nM final concentration) was used. Killing curves were performed for 3 piperacillin/tazobactam resistant strains chosen for their different range of MIC values to piperacillin/tazobactam (128, 256 and 512 mg/L, respectively). Analyses were performed in duplicate using a concentration of antibiotic of 16 mg/L. The addition of PEI (250 nM) increased the susceptibility of piperacillin/tazobactam (from 8 to 32 folds) in all the strains tested. Moreover, in all the strains tested piperacillin/tazobactam in association with PEI showed a bactericidal activity before 6 h. Intrinsic resistance of P. aeruginosa affects susceptibility to penicillins and is believed to entail broad spectrum impermeability. Different theories suggest that some degree of structure or organization exists in the periplasm of P. aeruginosa and that this modulates both the level of intrinsic, impermeability-determined resistance expressed by a strain and the efficiency with which the β-lactamase can contact and destroy the incoming β-lactamase molecules. The explanation of this behaviour could be that PEI determines an increased permeability of the outer membrane of P. aeruginosa; tazobactam inhibits β-lactamase allowing the antibacterial activity of piperacillin. In conclusion our data suggest that PEI can be expected to act as a vehicle for experimental drug delivery into cells, and as a potentiating agent for antibacterial agents that are normally excluded by Gram-negative bacteria.