A total of 489 clinical isolates of Pseudomonas aeruginosa was investigated for metallo--lactamase (MBL) production. Molecular analysis detected a bla VIM-1 gene in the chromosome of one isolate and a bla VIM-2 gene carried on the plasmid in seven isolates. Moreover, we showed that an initial screening by combined susceptibility testing of imipenem and ceftazidime followed by a confirmatory EDTA combination disk test represents a valid alternative to the molecular investigation of MBL genes, making MBL detection possible in routine diagnostic laboratories.Metallo--lactamase (MBL)-producing Gram-negative bacteria are an increasing public health problem worldwide because of their resistance to all -lactams except aztreonam (3). MBL genes are typically part of an integron and are either carried on transferable plasmids or are part of the bacterial chromosome (28). The most common transferable MBL families include the VIM-, IMP-, GIM-, SPM-, and SIM-type enzymes which have been detected primarily in Pseudomonas aeruginosa but were also found in other Gram-negative bacteria, including nonfermenters and members of the family Enterobacteriaceae (22). Recently, two new subgroups of MBLs, designated NDM-1 and DIM-1, were identified in a clinical isolate of Klebsiella pneumoniae in India and in a clinical isolate of Pseudomonas stutzeri in the Netherlands, respectively (21, 31).In most studies, reduced susceptibility to imipenem has been adopted as the sole criterion for further phenotypic or molecular investigations in order to detect MBLs (11,19,20,23). However, this criterion seems to be suboptimal, as it does not allow exclusion of isolates characterized by the loss of the OprD porin, the most common mechanism of resistance to imipenem in P. aeruginosa (14,22). Moreover, several phenotypic tests for detecting MBLs, such as the MBL Etest (20, 25), EDTA combination disk test (20,25,30), EDTA disk synergy test (10), and imipenem lysate MBL assay (27), have been developed and evaluated. However, the performance of each of these tests seems to be strongly affected by the local rate of MBL-producing isolates.In Germany, VIM-, and GIM-type enzymes in P. aeruginosa isolates have already been detected (1,8,26). Recently, Elias et al. described a nosocomial outbreak caused by a bla VIM-2 -positive P. aeruginosa in patients of the Department of Urology of the university hospital of the University of Würzburg, Würz-burg, Germany, between November and December 2007 (4). However, to the best of our knowledge, no systematic surveys of the occurrence of MBLs in clinical isolates of P. aeruginosa have been conducted in Germany so far. Accordingly, this study was designed with the following aims: (i) to develop improved screening criteria for the detection of MBL-producing P. aeruginosa; (ii) to determine the proportion of MBL-producing isolates in clinical isolates of P. aeruginosa in the university hospital of the University of Würzburg in Germany; (iii) to assess the relatedness of MBL-producing isolates; (iv) to determine the lo...