MINIPLASMID DERIVED FROM LISTERIA-MONOCYTOGENES MULTIRESISTANCE PLASMID PWDB100 UPON CONJUGAL TRANSFER INTO STAPHYLOCOCCUS-EPIDERMIDIS CARRIES CHLORAMPHENICOL RESISTANCE GENE IDENTICAL WITH STAPHYLOCOCCAL GENE

Multiresistance in Listeria monocytogenes has been found only in three isolates in France and Switzerland and is caused by a large conjugati ve plasmid. The Swiss isolate LM2, carries plasmid pWDB100 which is tr ansferable 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 derivativ e of pWDB100, designated pWDB100/m. The large plasmid could be elimina ted, and a derivative resistant to only chloramphenicol (Cm) and harbo ring only pWDB100/m was obtained. The sequence of pWDB100/m showed 267 9 bp. Two open reading frames coding for putative proteins of 307 and 215 amino acids were detected. By sequence comparison these proteins t urned out to be a Rep (replication initiation protein) and a Cat (chlo ramphenicol-acetyltransferase), respectively. The Cat showed 37.1 to 1 00% amino acid-identity to Cats from other organisms. The closest rela tionship was with the pC223-family of small rolling circle replication plasmids mediating Cm-resistance in staphylococci. Sequences surround ing the cat gene in pWDB100/m were organized in short segments with va rious degrees of homology to other plasmids that contained either Cm- or other resistance-determinants. Adjacent to three such homology-disc ontinuities there were short and thymine-rich segments that were simil ar to each other and contained several G(A)(A)TTT-motifs.