Hospital outbreak of vancomycin-resistant enterococci caused by a single clone of Enterococcus raffinosus and several clones of Enterococcus faecium

Abstract

A mixed outbreak caused by vancomycin-resistant Enterococcus raffinosus and Enterococcus faecium carrying the vanA gene was analysed. The outbreak occurred in a large hospital in Poland and affected 27 patients, most of whom were colonised, in three wards, including the haematology unit. The E. raffinosus isolates had a high-level multiresistant phenotype and were initially misidentified as Enterococcus avium; their unambiguous identification was provided by multilocus sequence analysis. The molecular investigation demonstrated the clonal character of the E. raffinosus outbreak and the polyclonal structure of the E. faecium isolates. All of the isolates carried the same Tn1546-like element containing an IS1251-like insertion sequence, located on a c. 50-kb conjugative plasmid. One of the E. faecium clones, found previously to be endemic in the hospital, was probably the source of the plasmid. The results of the study suggest that difficulties in identification may have led to an underestimate of the importance of E. raffinosus in vancomycin-resistant enterococci (VRE) control strategies.