Comparison of enterococcal and staphylococcal beta-lactamase plasmids.

In Staphylococcus aureus, beta-lactamase (Bla) is typically associated with transducible plasmids that also encode metal resistance or with the more recently described conjugative, gentamicin resistance plasmids. The beta-lactamase gene of Enterococcus faecalis, which has been shown to be highly homologous to bla from the S. aureus plasmid p1258, is also encoded on conjugative, gentamicin resistance plasmids. To determine if the enterococcal Bla-encoding plasmids are similar to those from staphylococci, Bla-encoding plasmids from both species were compared by restriction endonuclease digestion followed by hybridization to Bla and gentamicin resistance gene probes and to one of the enterococcal Bla-encoding plasmids. Although similarities were noted in the restriction endonuclease digestion patterns among the transducible staphylococcal plasmids (pI524, pI258, and pII147) and among the conjugative Gmr staphylococcal plasmids, neither of these restriction patterns was similar to those of the enterococcal plasmids. Although the enterococcal plasmids showed extensive similarities by hybridization, the staphylococcal plasmids showed little or no similarity to the enterococcal plasmids except for bla and the gentamicin resistance genes. Whether enterococci acquired a different type of Bla-encoding plasmid or whether bla was acquired by transposition or by homologous recombination into enterococcal plasmids is unknown.