Pseudomonas aeruginosa strains harbouring an unusual blaVIM-4 gene cassette isolated from hospitalized children in Poland (1998-2001).

During 1997-2001, 151 isolates of imipenem-resistant Pseudomonas aeruginosa were obtained from clinical specimens taken from children hospitalized in Warsaw, Poland. These strains were investigated further to determine the mechanism of resistance. The strains were analysed by a combination of genotyping and PCR-based strategies. Eleven of these strains were found to contain the metallo-beta-lactamase (M beta L) gene bla(VIM-4). The first strain appeared in 1998, and P. aeruginosa strains harbouring this M beta L have become endemic in this hospital since then. All P. aeruginosa strains belonged to serotype O:6, and PFGE analysis revealed four different patterns and three sub-types. All 11 M beta L-producing strains contained an identical class 1 integron with the usual 5' and 3' conserved sequences. The integron included two resistance cassettes, aacA4 in the first position and the bla(VIM-4) cassette in the second position. The bla(VIM-4) gene included an unusual direct repeat of 169 bp of the 3' portion of the bla(VIM-4) gene. An unusual bla(VIM-4) M beta L has become endemic in P. aeruginosa isolates infecting Polish children hospitalized on surgical wards. The formation of this unusual bla(VIM-4) gene cassette could be explained by a mechanism involving deletion of a segment of an ancestral tandem repeat of bla(VIM-4) via slipped strand replication, mediated by a combination of polymerase and integrase.