Nosocomial Infections Caused by Multidrug-Resistant Isolates of Pseudomonas putidaProducing VIM-1 Metallo-ß-Lactamase

ABSTRACTSuccessful carbapenem-based chemotherapy for the treatment of Pseudomonasinfections has been seriously hindered by the recent appearance of IMP- and VIM-type metallo-ß-lactamases, which confer high-level resistance to carbapenems and most other ß-lactams. Recently, multidrug-resistant Pseudomonas putidaisolates for which carbapenem MICs were =32 µg/ml were recovered from cultures of urine from three inpatients in the general intensive care unit of the Ospedale di Circolo, Varese, Italy. Enzyme assays revealed production of a metallo-ß-lactamase activity, while molecular analysis detected in each isolate a blaVIM-1determinant carried by an apparently identical medium-sized plasmid. Conjugation experiments were unsuccessful in transferring the ß-lactamase determinant to Escherichia colior Pseudomonas aeruginosa. Macrorestriction analysis by pulsed-field gel electrophoresis demonstrated that the isolates were of clonal origin. PCR mapping and sequencing of the variable region of the plasmid-borne class 1 integron carrying the blaVIM-1determinant (named In110) showed that the blaVIM-1-containing cassette was identical to that previously found in strains of different species from other Italian hospitals and that the cassette array of In110 was not identical but clearly related to that of In70 (a blaVIM-1-containing plasmid-borne integron from an Achromobacter xylosoxidansisolate), pointing to a common origin of this cassette and to a related evolutionary history of their cognate integrons.