Composition and antibiotic resistance profile of microcosm dental plaques before and after exposure to tetracycline.

The aim of this study was to investigate the effects of tetracycline administration on the viability and antibiotic resistance profiles of microcosm dental plaques. A constant depth film fermenter was used to generate multi-species biofilms, which were grown for 216 h before tetracycline was added. The composition of the microcosm plaques was determined by viable counting on selective and non-selective media. The prevalence of antibiotic-resistant organisms was determined on antibiotic-containing media. Before administration of tetracycline, the biofilms had a total viable anaerobic count of 7 x 10(7) cfu per biofilm. They contained 7% lactobacilli, 19% streptococci and 2% Actinomyces spp. Immediately after pulsing with tetracycline, the composition of the biofilms changed and they consisted of 30% lactobacilli, 1.5% streptococci and 3% Actinomyces spp., with a total anaerobic count of 1 x 10(7) cfu per biofilm. The pre-valence and composition of the antibiotic-resistant microflora changed dramatically after the addition of tetracycline, with the proportion of the microflora displaying resistance to tetracycline increasing from 6% to 45%. Corresponding changes in the proportions of the microflora displaying resistance to other antibiotics were as follows: 5-28% for erythromycin, 1-5% for vancomycin and 0.4-3% for ampicillin. The results of this study have shown that the addition of tetracycline to microcosm dental plaques alters their composition and enriches for bacteria resistant to tetracycline and other unrelated agents.