Objectives: 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.
Methods: The strains were analysed by a combination of genotyping and PCR-based strategies.
Results: 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.
Conclusions: 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.