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

Abstract

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-β-lactamase (MβL) gene bla_VIM-4. The first strain appeared in 1998, and P. aeruginosa strains harbouring this MβL 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 MβL-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 MβL 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.