Enterococcus faecalis (E. faecalis) ATCC51299 used as a control in screening for high-level aminoglycoside resistance in enterococci was observed to transfer plasmids to the clinically isolated E. faecalis strain 4437 in a broth medium. From a mixed culture of strain ATCC51299 and strain 4437, transconjugants were obtained not only from the planktonic phase but also from biofilm. Plasmid transfer frequency was approximately one order higher among cells in the biofilm than among those in the

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... ktonic phase. When the biofilm formed by the recipient strain E. faecalis 4437 was inoculated with the donor strain E. faecalis ATCC51299, correlations were observed between the number of donor cells and transconjugants, and the number of donor cells and transfer frequency in the biofilm (r = 0.99, 0.98, respectively). In addition, the transfer frequency in biofilm remained constant after 9-hr incubation, while transfer frequency in the planktonic phase decreased with incubation time. These results suggest that the biofilm formed by enterococci on surfaces gives rise to efficient gene transfer.