Demonstration of in vivo transfer of doxycycline resistance mediated by a novel transposon.

OBJECTIVES The aim of this study was to investigate the transfer of bacterial doxycycline resistance between oral bacteria in subjects receiving systemic doxycycline for the treatment of periodontitis. PATIENTS AND METHODS Streptococci were cultured before and after treatment from the subgingival plaque of two patients with periodontitis, genotyped and investigated for the presence of antimicrobial resistance determinants and conjugative transposons. RESULTS In one subject, a strain of Streptococcus sanguinis resistant to doxycycline was a minor component of the pre-treatment streptococcal flora but dominated post-treatment. In a second subject, a strain of Streptococcus cristatus, which was sensitive to doxycycline before treatment, was found to have acquired a novel conjugative transposon during treatment, rendering it resistant to doxycycline and erythromycin. The novel transposon, named CTn6002, was sequenced and found to be a complex element derived in part from Tn916, and an unknown element which included the erythromycin resistance gene erm(B). A strain of Streptococcus oralis isolated from this subject pre-treatment was found to harbour CTn6002 and was therefore implicated as the donor. CONCLUSIONS This is the first direct demonstration of transfer of antimicrobial resistance carried on a conjugative transposon between oral bacteria during systemic antimicrobial treatment of periodontitis in humans.