Multifocal detection of multidrug-resistant Pseudomonas aeruginosa producing the PER-1 extended-spectrum β-lactamase in Northern Italy

Forty-four nonreplicate clinical isolates of Pseudomonas aeruginosa that were resistant to extended-spectrum cephalosporins (ceftazidime and cefepime) and aztreonam, that putatively produced an acquired extended-spectrum P-lactamase (ESBL), according to the results of a double-disk synergy test, and that had been involved in nosocomial outbreaks were obtained from six different hospitals in northern Italy and screened for the presence of bla(PER), ESBL determinants. Twenty isolates, associated with nine independent outbreaks that occurred in five hospitals in the Milan area and its surroundings during 1995-2000, were found to carry an acquired bla(PER-1) gene. PER-1 producers representative of the nine outbreaks exhibited a multidrug resistance (MDR) phenotype, including resistance to extended-spectrum cephalosporins, aztreonam, meropenem, aminoglycosides, and in most cases, imipenem and ciprofloxacin. An analysis of macrorestriction profiles of their genomic DNAs by pulsed-field gel ellectrophoresis revealed an overall clonal diversity of the PER-1 producers, although interhospital clonal spread was also observed. The bla(PER-1) gene was not transferable and appeared to be chromosomally located. An analysis of the EcoRI and EcoRV restriction fragment length polymorphisms of the blaPER-1 locus revealed identical patterns for all isolates, and the characterization of a 1.9-kb region containing bla(PER-1) revealed a conserved structure in representatives of the various clonal lineages. The present findings indicate that MDR P. aeruginosa clones producing the PER-1 ESBL are endemic to this area of northern Italy, where they have been circulating since the mid-1990s and have been associated with several nosocomial outbreaks.