Molecular evolution of metallo-beta-lactamase-producing Pseudomonas aeruginosa in a nosocomial setting of high-level endemicity

An outbreak of multidrug-resistant P. aeruginosa strains producing VIM-type metallo-beta-lacta-mases (MBLs) has occurred in an Italian hospital since 2000. In this work we characterized 128 carbapenem-resistant isolates collected from that hospital from 2000 to 2002. Genotyping showed that most VIM-positive isolates belonged to two different clonal lineages, producing either a VIM-1- or a VIM-2-like MBL. The 86 clonally related isolates carrying a blaVIM-1-like gene on an In70-like integron were clearly related to a VIM-1-positive P. aeruginosa clone circulating in various Italian hospitals since the late 1990s. In the VIM-2-like positive clone, the MBL gene was carried by an unusual class 1 integron, named In71, lacking the 3’ conserved sequence region typical of sul1-associated integrons. A retrospective investigation did not reveal the presence of strains related to any of the VIM-producing isolates earlier than 1997.