[Transfer of bacterial drug resistance factor in artificial dental plaque]

Transfer of drug resistance factor was examined using so-called artificial dental plaque. Clinically isolated specimens of Enterococcus faecalis showing various kind of drug resistance were used as the donors. Streptococcus mutans BHT strain was used as the recipient and the plaque former. S. mutans formed artificial dental plaque on the surface of glass sticks after 7 to 10 day cultivation in brain heart infusion medium containing 5% sucrose. Each strain of E. faecalis was added to the plaque culture and co-cultured further 24 hours. Colonies were formed on drug-containing plates by 5 different mixed cultures with the frequencies of 10(-5)-10(-6) (kanamycin, KM) and 10(-4)-10(-8) (erythromycin, EM). Since these values were higher than that of the mutation frequency, we conclude that the resistant sublines were formed by a genetic transfer. Plasmid DNA derived from some transferred sublines was electrophoreted and the patterns were compared with those from the donor and the recipient. One KM-resistant subline showed a new band at the point of 40 kb which was not found in the plasmid DNA from the donor. It is suggested that the plasmid containing KM-resistance gene was modified molucularly. Sublines without new plasmid band were obtained also. There may be technical problems with it, or the transferred gene may be incorporated in a chromosome. Considering above results, the effect of bacterial transferable genes on oral hygiene were discussed.