Chromosomal DNA restriction endonuclease digestion patterns of β- lactamase-producing Enterococcus faecalis isolates collected from a single hospital over a 7-year period

Twenty-three β-lactamase (β-lac)-producing, highly gentamicin- resistant Enterococcus faecalis isolates collected over a 7-year period from the same hospital were examined by pulsed-field gel electrophoresis of SmaI- digested genomic DNA. The β-lac⁺ isolates appeared to form a single clonal group, which had been previously designated the mid-Atlantic pattern. Eleven variations of the mid-Atlantic clone, differing by one to six bands, were identified; some of the changes were likely due to plasmid bands. However, a number of isolates had indistinguishable patterns, including some recovered over a 4-year period. There was a surprising lack of movement of the β-lac determinant to other strains, although this trait was transferable in vitro by conjugation. We conclude that a single clone (the mid-Atlantic clone) of β-lac⁺ E. faecalis has remained endemic in this hospital for at least 7 years. The reason(s) for the apparent lack of spread to other strains of E. faecalis is unknown.