Characterization of tetracycline resistance in Actinobacillus actinomycetemcomitans

Comparison of susceptibility data for Actinobacillus actinomycetemcomitans has been difficult because of the lack of standard susceptibility testing conditions. In this study, minimum inhibitory concentration to tetracycline was evaluated by comparing different media, air conditions and incubation times. Ten of 22 (45%) A. actinomycetemcomitans isolated from periodontally diseased sites grew on media supplemented with 4 micrograms per ml of tetracycline, but minimum inhibitory concentrations ranged from 0.125 to 8 micrograms/ml depending on the media and condition used. The best results were obtained with brain heart infusion agar (Difco Laboratories, Detroit MI) incubated in 5% CO2 for 48 h. Eighteen (82%) of the A. actinomycetemcomitans isolates hybridized with the Tet B determinant. The Tet B determinant was transferable between A. actinomycetemcomitans isolates as well as a Haemophilus influenzae recipient and appears to be associated with conjugative plasmids.