Excision of a conjugative plasmid from the staphylococcal chromosome

Staphylococcus aureus isolate WBG1003 resistant to benzyl penicillin, cadmium, arsenate and streptomycin harbours two plasmids of 38.8 (pWBG621) and 4.4 (pWBG625) kb. In conjugation experiments two types of streptomycin-resistant transconjugants were obtained; one carried a 4.4-kb plasmid and the other, a 34.5-kb and a 4.4-kb plasmid. The 34.5-kb plasmid (pWBG620) has been found to be conjugative and able to mobilise non-conjugative plasmids. It has no detectable resistance phenotype and has not been detected in WBG1003 nor in the recipient used in the conjugation experiments. Restriction endonuclease analysis and DNA-DNA hybridisation have revealed that pWBG620 is unrelated to pWBG621 present in strain WBG1003. The data presented indicate that pWBG620 is in the chromosome of strain WBG1003 and that it excises during conjugation.