A new outbreak of VIM-1-producing Pseudomonas aeruginosa in the same site as the first VIM description

Abstract number: 1134_01_39

Mazzariol A., Bahar G., Konkan R., Fontana R., Cornaglia G.

Objectives:  

Pseudomonas aeruginosa strain PSE VR143/97 was responsible for an outbreak in the Verona University Hospital in 1997, this being the first world occurrence of a VIM-type enzyme. Between February 2003 and July 2004, 14 strains of P. aeruginosa highly resistant to carbapenems, 12 from the ICU and two from the hematology unit, were isolated from the same hospital. All strains were investigated for their possible production of metallo-beta-lactamase.

Methods:  

Antimicrobial susceptibility testing was performed by both diffusion and microdilution and interpreted according to the latest NCCLS documents. The presence of carbapenemase was screened by means of the MBL-etest, and further investigated spectrophotometrically by following the crude sonic extract hydrolysis of imipenem and its inhibition by EDTA. PCRs were performed with either bla(VIM) or bla(IMP) primers. PFGE, isoelectrofocusing, and sequencing were carried out according to standard procedures. The integron carrying the bla gene was also studied by both PCR and sequencing.

Results:  

All strains proved resistant to all beta-lactams tested, with the sole exception of aztreonam, as well as to ciprofloxacin, gentamicin, amikacin, and tobramycin. The MBL-etest was suggestive of an MBL in all cases. Imipenem hydrolysis confirmed that imipenem was hydrolysed at a rate of 1 × 10^­8 mol/min/mg, whereas in the presence of EDTA 2 mM the kinetics was 1 × 10^­9 mol/min/mg. The isoelectrofocusing showed a band at 5.3, i.e. the same level as VIM-1 from the index strain PSE VR143/97. PCRs performed with either bla(VIM) or bla(IMP) primers yielded positive results only with the bla(VIM) primers; a specific PCR performed with the bla(VIM-1) primers yielded a product of about 800 bp which, after sequencing, was found to code for VIM-1. The strains showed PFGE patterns that were either identical or correlated to one another but were different from the pattern of index strain PSE VR143/97 which was responsible for the outbreak in 1997 in the same hospital.

Conclusions:  

All strains carried a VIM-1 enzyme, and seemed to be derived from the same clone. They sharply differed from the index strain responsible for the 1997 outbreak. These results enabled us to exclude the persistence of PSE VR143/97 in the two wards of the Verona University Hospital, but showed that the existing infection control procedures were not sufficient to avoid the diffusion and long-term persistence of a new VIM-1-bearing P. aeruginosa.