Clonal diversity and metallo-β-lactamase production in clinical isolates of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is a nosocomial pathogen with an intrinsic broad-spectrum resistance to β-lactam compounds and other antibacterial agents. It produces two chromosomal β-lactamases: a clavulanic acid-sensitive class A (L2) and a tetrameric carbapenemase (L1 or BlAS). We screened 40 S. maltophilia multidrug-resistant clinical isolates recovered between 1995 and 1998 in the Varese Hospital (Italy) for the presence of the metallo-β-lactamase. The isolates were investigated by phenotypic profiling (enzymatic activity and antibiotic resistance pattern) and molecular methods such as PCR and pulsed-field gel electrophoresis (PFGE) to reveal intraspecies diversity. For the tested S. maltophilia strains, we showed that the β-lactamase production could be induced by the presence of imipenem (50 μg/ml) in the culture media. Addition of 1 mM dipicolinic acid completely inhibited the hydrolysis of imipenem but decreased that nitrocefin in a strain-dependent manner. Full activity of crude extract towards imipenem could be restored by addition of 1 mM ZnCl2. Finally, the gene encoding the carbapenem-hydrolyzing β-lactamase from S. maltophilia ULA-511 and 39/95, a clinical strain, were isolated and sequenced. These two strains have a different profile of multidrug resistance. The two metallo-β-lactamases were found to be isologous. The difference of sensitivity of these two strains was associated to the level of production of the metallo-β-lactamase.