Objectives: To investigate the first Italian outbreak of bloodstream infections caused by multidrug-resistant (MDR) Klebsiella pneumoniae producing metallo-beta-lactamase (MBL), which occurred in three wards of one large tertiary-care hospital in Genoa, Italy, from September 2004 to March 2005.
Methods: MBL production was screened by an imipenem-EDTA disc synergy test and confirmed by a conventional hydrolysis test. Antibiotic susceptibility was determined by broth microdilution or disc diffusion. PFGE was used to study the genetic relatedness of isolates. PCR and sequencing were carried out to identify the beta-lactamase genes and to analyse the genetic context of the MBL gene. Outer membrane protein (OMP) profiles were analysed by SDS-PAGE.
Results: Nine cases of bloodstream infections caused by an MDR strain of K. pneumoniae producing the VIM-1 MBL and the SHV-5 extended-spectrum beta-lactamase (ESBL) were identified. The isolates exhibited various carbapenem resistance levels (imipenem MICs ranged from 4 to 64 mg/L) and were resistant to other beta-lactams, fluoroquinolones, trimethoprim/sulfamethoxazole and chloramphenicol. The isolate with the highest imipenem MIC also lacked the k36 OMP. The bla(VIM-1) gene cassette was part of the variable region of a class 1 integron that also included an aac(6')-IIc cassette. The ESBL and MBL genes were transferable by conjugation.
Conclusions: This is the first report on the emergence of an MDR strain of K. pneumoniae producing the VIM-1 MBL, causing an outbreak of bloodstream infections in an Italian hospital. The strain evolved through OMP alterations generating a mutant with increased carbapenem resistance.