Miniplasmid Derived from Listeria monocytogenes Multiresistance Plasmid pWDB100 upon Conjugal Transfer into Staphylococcus epidermidis Carries Chloramphenicol Resistance Gene Identical with Staphylococcal Gene

Abstract

Multiresistance in Listeria monocytogenes has been found only in three isolates in France and Switzerland and is caused by a large conjugative plasmid. The Swiss isolate LM2, carries plasmid pWDB100 which is transferable to several species including Staphylococcus epidermidis . In many S. epidermidis (pWDB100) transconjugants a second plasmid of 2.6 kb in length was discovered. It was shown to be an excision derivative of pWDB100, designated pWDB100/m. The large plasmid could be eliminated, and a derivative resistant to only chloramphenicol (Cm) and harboring only pWDB100/m was obtained. The sequence of pWDB100/m showed 2679 bp. Two open reading frames coding for putative proteins of 307 and 215 amino acids were detected. By sequence comparison these proteins turned out to be a Rep (replication initiation protein) and a Cat (chloramphenicol-acetyltransferase), respectively. The Cat showed 37.1 to 100% amino acid-identity to Cats from other organisms. The closest relationship was with the pC223- family of small rolling circle replication plasmids mediating Cm-resistance in staphylococci. Sequences surrounding the cat gene in pWDB100/m were organized in short segments with various degrees of homology to other plasmids that contained either Cm- or other resistance-determinants. Adjacent to three such homology-discontinuities there were short and thymine-rich segments that were similar to each other and contained several G(A)(A)TTT-motifs.