Nosocomial Outbreak Caused by Multidrug-Resistant Pseudomonas aeruginosa Producing IMP-13 Metallo-β-Lactamase

An outbreak of Pseudomonas aeruginosa showing a multidrug-resistant (MDR) phenotype (including carbapenems, ceftazidime, cefepime, gentamicin, tobramycin, and fluoroquinolones) was observed, during a 5-month period, in a general intensive care unit of a large tertiary care and clinical research hospital in southern Italy. The outbreak involved 15 patients, with a total of 87 isolates, mostly from lower respiratory tract specimens. Analysis of isolates involved in the outbreak revealed production of metallo-β-lactamase (MBL) activity, and genotyping by pulsed-field gel electrophoresis of genomic DNA digested by SpeI revealed clonal relatedness among isolates. Molecular analysis of the MBL determinant showed the presence of a blaIMP-13 gene carried on a gene cassette inserted in a class 1 integron which also contained an aacA4 aminoglycoside resistance cassette encoding an AAC(6′)-Ib enzyme. The blaIMP-13-containing integron and its genetic environment appeared to be similar to those found in P. aeruginosa isolates producing IMP-13 from a hospital in Rome. The blaIMP-13 gene was not transferable by conjugation and was apparently carried on the chromosome. The outbreak was coincidental with a shortage of nursing personnel, and resolution was apparently associated with reinstatement of nursing personnel and reinforcement of general infection control practices within the intensive care unit. To our best knowledge this is the first description of a nosocomial outbreak of relatively large size caused by an IMP-producing gram-negative pathogen in Europe.