Clonal relatedness and conserved integron structures in epidemiologically unrelated Pseudomonas aeruginosa strains producing the VIM-1 metallo-β-lactamase from different Italian hospitals

Three epidemiologically independent Pseudomonas aeruginosa isolates, representative of the first VIM-1 metallo-β-lactamase producers detected at three different hospitals in northern Italy, were investigated to determine their genomic relatedness and to compare the structures of the genetic supports for the VIM-1 determinants. The three isolates, all of serotype O11, appeared to be clonally related according to the results of genotyping by macrorestriction analysis of genomic DNA by pulsed-field gel electrophoresis and random amplification of polymorphic DNA. Investigation of the genetic support for the blaVIM-1 determinant revealed that it was carried on identical or almost identical integrons (named In70.2 and In70.3) located within a conserved genomic context. The integrons were structurally related to In70 and In110, two plasmid-borne blaVIM-1-containing integrons from Achromobacter xylosoxidans and Pseudomonas putida isolates, respectively, from the same geographic area (northern Italy) and were found to be inserted close to the res site of a Tn5051-like transposon, different from any of those described previously, that was apparently carried on the bacterial chromosome. The present findings suggest that the three VIM-1-producing isolates are members of the same clonal complex which have been spreading in hospitals in northern Italy since the late 1990s and point to a common ancestry of their bla VIM-1-containing integrons.