Diversity and stability of Plasmids from glycopeptide-resistant Enterococcus faecium (GRE) isolated from pigs in Denmark

In this paper, we examine the plasmid variation between a subset of unrelated GRE isolated from pigs in Denmark between 1995 and 2001 (five from each of the years). The isolates were tested with PFGE, plasmid RFLP, and subsequently Southern blotting with an IS1216V probe. Of the 35 isolates, 31 belonged to the same PFGE type (type 1), and the last four belonged to a completely different PFGE type (type II). All 35 isolates contained the same type of large plasmid (approximate size of 150-200 kb), which could be divided into eight different variant types (V0 to V7). Most variance among the plasmid was seen at the end of the 7-year period, coinciding with the ban in 1998 of the macrolide tylosin as growth promoter for pig production. The stability of the plasmid in its original host was compared with stability of the same plasmid in BM4105RF, when both strains were maintained in liquid cultures without the presence of antibiotics for 1,100 generations. Here, the plasmid proved far more stable in its original host environment than in the new host.