Molecular analysis of Tn1546-like elements mediating high-level vancomycin resistance in Enterococcus gallinarum

Four Enterococcus gallinarum isolates, all highly resistant to vancomycin, were studied in order to investigate their relationship and to gain insight into the molecular events responsible for their acquired resistance.

Extensive molecular analysis was performed to compare the four E. gallinarum isolates and their Tn1546-like elements.

The four strains had very similar random amplified polymorphic DNA (RAPD) patterns and different but related PFGE profiles. Genotypic analysis demonstrated that all carried both vanC-1 and vanA genes. Using a vanA probe, no hybridization was detected to plasmid DNA, whereas hybridization to different SmaI fragments of the four strains was obtained with total DNA. Amplification and sequencing experiments showed that all four strains carried a Tn1546-like element that contained the orf2, vanR, vanS, vanH, vanA and vanX genes and was flanked at both ends by oppositely oriented IS1216V sequences. On the left side of the vanA cluster, all lacked IRL, and all had, upstream from orf2, 1029 bp of the 3' end of orf1. On the right side, one of the strains lacked vanY, vanZ and IRR, whereas in one of the other three there was an IS1542 element inserted within the vanZ gene. In one strain, an additional IS1216V element was inserted in the intergenic region vanX-vanY.

This is the first study providing a molecular analysis of chromosomal Tn1546-like elements (possibly composite transposons) associated with high-level vancomycin resistance in human and animal strains of E. gallinarum. These molecular findings, together with those from PFGE and RAPD, suggest that the four E. gallinarum isolates are related and might have a common ancestor.