IN-VITRO ACTIVITY OF PIPERACILLIN TAZOBACTAM AGAINST 615 PSEUDOMONAS-AERUGINOSA STRAINS ISOLATED IN INTENSIVE-CARE UNITS/

From May 1996 to September 1997, 615 Pseudomonas aeruginosa strains is olated from patients in intensive care units collected from different Italian laboratories were studied. The susceptibility of piperacillin/ tazobactam, in comparison with other antipseudomonal antibiotics, to t heir NCCLS breakpoints was evaluated: amikacin 79.6%, carbenicillin 67 .0%, ceftazidime 73.4%, ciprofloxacin 55.8%, imipenem 64.1%, piperacil lin 88.1%, piperacillin/tazobactam 92.4% and ticarcillin/clavulanic ac id 69.0%. Seventy-three strains were selected because of their resista nce to piperacillin and the mechanisms underlying such a resistance we re investigated. Isoelectric focusing and hydrolysis assays revealed t he presence of 15 plasmid-mediated p-lactamases. Chromosomal beta-lact amase derepression was demonstrated in 34 isolates. The remaining 24 p iperacillin-resistant strains did not produce beta-lactamases and an ' intrinsic mechanism' of resistance was inferred. The piperacillin/tazo bactam combination restored resistance in 25 piperacillin strains. Nin e of these were derepressed for chromosomal beta-lactamase, 8 showed i mpermeability and 8 showed plasmid enzymes.