Molecular epidemiology of β-lactamase-producing enterococci

Plasmids from the first six reported β-lactamase-producing (Bla⁺) enterococci were compared for genetic relatedness. Bla⁺ enterococcal plasmids from strains isolated in Houston, Tex.; Philadelphia, Pa.; Connecticut; and Pittsburgh, Pa., had heterogeneous HaeIII and MspI-ClaI restriction endonuclease digestion patterns. A staphylococcal β-lactamase probe hybridized to all six Bla⁺ enterococcal plasmids, but hybridization was detected on different HaeIII and MspI-ClaI fragments of the six plasmids. An enterococcal gentamicin resistance (Gm(r)) probe hybridized to a common 3.9-kilobase HaeIII fragment from the five Gm(r) plasmids. The Houston plasmid was cross-hybridized to the other five strains, and moderate to extensive homology was demonstrated. Bla⁺ enterococcal plasmids from a broad geographic range are heterogeneous with respect to size and restriction endonuclease digestion patterns but contain homologous genetic material, including Bla⁺ and Gm(r) determinants.