Genotypic characterization of a nosocomial outbreak of VanA Enterococcus faecalis

Abstract

Despite growing concern about vancomycin-resistant enterococci (VRE) as nosocomial pathogens, especially in the United States, in Italy VRE still represent an uncommon and occasional experience for most diagnostic laboratories. We report a genotypic characterization of the first reported nosocomial outbreak of VRE in Italy. Some experiments, including plasmid analysis and pulsed-field gel electrophoresis (PFGE) assays, aimed at investigating the genetic relatedness of the VRE isolates. Other experiments, based on hybridization and polymerase chain reaction (PCR) assays, aimed at characterizing the vancomycin resistance determinants. Over a 6-month period, 21 VRE, all identified as Enterococcus faecalis, were isolated from eight patients (all treated earlier with glycopeptide antibiotics) in a neurosurgical intensive care unit. All isolates had the same biochemical profile and antibiotic susceptibility pattern, including high-level resistance to aminoglycosides and vancomycin and teicoplanin MICs of 256 and 128 micrograms/ml, respectively. Three plasmids, one strongly hybridizing with a vanA probe, were detected in all but the last of the 21 VRE isolates. The last isolate of the cluster lacked the smallest of the three plasmids. Similar restriction profiles were obtained after plasmid DNA digestion with several endonucleases, with minor differences appreciated only in the first and last isolates. Analysis of genomic DNA restriction fragment patterns by PFGE confirmed that the reported cluster of VRE isolations was due to a single nosocomial strain of E. faecalis, despite some modifications in plasmid DNA at the beginning and at the end of the outbreak. Completely different PFGE patterns were yielded by vancomycin-susceptible E. faecalis strains isolated during the same period from inpatients in the same intensive care unit. Hybridization experiments with vanA and vanS-vanH probes and DNA amplification assays using 14 PCR primer pairs specific for vanA cluster genes (vanR, vanS, vanH, vanA, and vanY), orf1, orf2, vanB, and vanC showed identical organization of resistance determinants in all epidemic VRE isolates. This organization appeared to be the same as that described for Tn1546 in VanA prototype strain E. faecium BM4147.