To the Editor: Acquired metallo-ß-lactamase determinants in Pseudomonas aeruginosa and other major bacterial pathogens are of concern for development of antimicrobial drug resistance. The carbapenemase and extended-spectrum cephalosporinase activity of metallo-ß-lactamases, as well as their resistance to ß-lactamase inhibitors, may severely limit the antimicrobial agents active against bacterial strains that produce such enzymes (1,2). Antimicrobial chemotherapy may become ineffective against P. aeruginosa strains with a multidrug-resistant phenotype that have acquired a metallo-ß-lactamase determinant.

We recently described a new acquired metallo-ß-lactamase determinant, bla_VIM, in a carbapenem-resistant P. aeruginosa clinical isolate (VR-143/97) from the University Hospital of Verona, Italy (3). This isolate was the index strain of an outbreak of bla_VIM-positive P. aeruginosa, which was caused both by strains that were clonally related to VR-143/97 and by clonally unrelated strains (4). bla_VIM is the second known metallo-ß-lactamase determinant that can spread among P. aeruginosa; the first was bla_IMP, which was detected in the early 1990s in nosocomial isolates of various Enterobacteriaceae, P. aeruginosa, and other nonfastidious gram-negative nonfermenters from the Far East (1,2,5-7) and, recently, in an Acinetobacter baumannii clinical isolate from Italy (8). Although completely unrelated at the sequence level, bla_VIM resembles bla_IMP in being carried on an integron-borne mobile gene cassette and in encoding an enzyme (VIM-1) with broad substrate specificity (3). Because of these properties, bla_VIM has the potential to become a dangerous resistance determinant.

An analysis of carbapenem-resistant P. aeruginosa from Italian hospitals since 1998 showed production of metallo-ß-lactamase, assayed as described (3), in five isolates from three hospitals in Italy. Two isolates (PPV-97 and PPV-108) were from the University Hospital of Pavia (PPV-97 was isolated in September 1998 from the urine of an inpatient in the neurosurgery department, and PPV-108 was isolated in November 1998 from a decubitus ulcer of an inpatient in the vascular surgery department); two (TS-832035 and TS-832347) were isolated in February 1999 from the University Hospital of Trieste (both from the blood of inpatients, in the intensive care unit and in the internal medicine department, respectively); and one (SAP-01/99) was isolated in September 1999 from the Rome University Hospital "Policlinico Umberto I" (from the blood of an inpatient in the vascular surgery department). Except for those from Pavia Hospital, where two departments share the same surgical unit, no epidemiologic relationship could be established among any of them or with those previously isolated in Verona (3,4).

The five isolates were highly resistant to carbapenems (MICs for imipenem and meropenem were >64 µg/ml) and to carbenicillin, ticarcillin, ticarcillin/clavulanate, piperacillin, piperacillin/tazobactam, mezlocillin, ciprofloxacin, gentamicin, tobramycin, and netilmicin. All five were also resistant to ceftazidime and cefepime, except for SAP-01/99, which had intermediate resistance to the above drugs. Some isolates retained susceptibility to aztreonam (PPV-97, TS-832035, and SAP-01/99) or amikacin (PPV-108, TS-832035, and TS-832347).

In a colony-blot hybridization assay (3), all the above isolates were recognized by a bla_VIM-specific probe consisting of an amplicon that contained the entire bla_VIM coding sequence (3). None were recognized by a probe specific for bla_IMP and consisting of a 0.5-kb HindIII fragment from the bla_IMP gene (8).

Our results indicate that circulation of carbapenem-resistant P. aeruginosa carrying bla_VIM metallo-ß-lactamase determinants, originally detected in one Italian hospital (3-4), could soon become widespread. The detection in different hospitals of bla_VIM-positive isolates that apparently were epidemiologically unrelated suggests that the environmental reservoir of bla_VIM-containing strains is relatively broad and that this novel determinant has potential relevance for the emerging phenomenon of carbapenem resistance in P. aeruginosa. Since we did not sequence the bla_VIM-related genes carried by the various isolates, we do not yet know whether they differ from that cloned from P. aeruginosa VR-143/97 (3). The five isolates described in this report are being characterized to ascertain their clonal relatedness and identify the sequences of their bla_VIM-related determinants. The recent appearance of this and other acquired metallo-ß-lactamases among P. aeruginosa and other gram-negative pathogens in Europe (8-10) underlines the need for systematic surveillance to monitor the spread of similar resistance determinants.

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