Presence and analysis of large plasmids in Oenococcus oeni

The use of a large-scale isolation technique to screen 30 Oenococcus oeni strains for extrachromosomal DNA led to the finding of large plasmids (ca. 40 kb) in most of the strains as well as the finding of small plasmids (2.5 to 4.5 kb) in 6 of the strains. The circular nature of the large plasmids was assessed by electrophoresis in ethidium bromide continuous gradient gels and the different conformations of these elements could be distinguished by three run types of pulsed-field gel electrophoresis (PFGE). Southern hybridization with PFG-electrophoresed DNA also suggested that the low extraction yields of these probably low-copy-number plasmids could be related to the generation of the OC conformation resulting from nicking during cell lysis. The results seem to indicate that the occurrence of large plasmids in O. oeni may be more frequent than is usually considered, although their presence is difficult to ascertain. Furthermore, spontaneous curing of these elements might occur. In fact, plasmid pOg25 (37 kb) was eventually not detected by DNA-DNA hybridization in its original host strain.