The aim of this study was to characterize two metallo-β-lactamases (MBLs)-producing Pseudomonas aeruginosa
clinical isolates showing meropenem susceptibility. Antimicrobial susceptibility was assessed by automated testing and Clinical and Laboratory typing was determined by pulsed field gel electrophoresis (PFGE) and sequencing. The location of blaIMP-16
was determine by plasmid electrophoresis, Southern blot and hybridization. Transcriptional levels of blaIMP-16
were determined by semi-quantitative real time PCR. The P. aeruginosa
isolates studied, Pa30 and Pa43, showed mipenem and meropenem susceptibility by automated testing. Ager dilution assays confirmed meropenem susceptibility whereas both isolates showed low level of imipenem re-sistance. Pa30 and Pa43 were phenotypically detected as MBL producers. PFGE revealed their clonal relatedness. blaIMP-16
was identified in both isolates, carried as a single cassette in a class 1 integron that was embedded in a plasmid of about 60-Kb. Pa30 and Pa43 overexpressed MexAB-OprM, MexCD-OprJ and MexXY-OprM efflux systems and showed basal transriptional levles of ampC
. MBL-producing P.aeruginosa
that are not resistant to meropenem amy represent a risk for therapeutic failure and act as silent reservoirs of MBL-encoding genes.