Objectives: A clinical isolate of Enterobacter cloacae exhibiting reduced susceptibility to imipenem and a positive EDTA-disc synergy test was studied for carbapenemase production.
Materials and methods: MICs were determined with standard procedures as well as using a higher inoculum. Isoelectric focusing of cell extracts was used for detection of beta-lactamases. PCR assays with primers specific for the bla(VIM) gene and the conserved segments of class 1 integrons and sequence analyses were carried out to identify the gene and to map the metallo-beta-lactamase encoding integron. Transferability of the gene was assessed with conjugation experiments using the filter mating technique. To identify the location of the bla(VIM-1) gene, Southern hybridization was carried out in genomic DNA using an internal fragment of the bla(VIM-1) gene as a probe, amplified by PCR.
Results: The isolate was resistant to extended-spectrum beta-lactams. The MICs of carbapenems were below the resistance breakpoints but rose above resistance breakpoints when an inoculum of 10(8) cfu/mL was used. Isoelectric focusing detected a beta-lactamase with a pI of 6.1, which exhibited imipenem-hydrolysing activity in a microbiological assay. Ceftazidime and imipenem resistance were not transferable by conjugation. PCR assays identified the bla(VIM-1) gene in the variable region of a class 1 integron which also carried the aac(6')-IIc gene. The bla(VIM-1) probe hybridized with an approximately 130 kb fragment of genomic DNA, suggesting a chromosomal location of the gene.
Conclusion: We describe a novel class 1 integron containing bla(VIM-1) and aac(6')-IIc genes in an E. cloacae clinical isolate.