ANTIBIOTIC-RESISTANCE OF THE SUBGINGIVAL MICROBIOTA FOLLOWING LOCAL TETRACYCLINE THERAPY

The antibiotic resistance of the subgingival microbiota was studied by 3 approaches. First, we assessed the ability of subgingival isolates taken following therapy to grow on media containing tetracycline (TC). Higher percentages of TC-resistant organisms appeared at TC fiber-treated periodontal sites and within the saliva 1 week after treatment as compared with pre-treatment levels. By 1 month, the percentage of TC-resistant organisms had returned to levels comparable to those seen before treatment. In the second approach, subgingival isolates taken following therapy were grown on media without antibiotics, and isolates were selected for Gram-stain and cell morphology determination. This study indicated that subgingival sites became colonized with gram-positive cocci in the same time period that an increase of TC-resistant isolates was observed in the first study. This may account for the transient increase in TC resistance, because many gram-positive cocci are intrinsically resistant to TC. In the third approach, the antibiotic resistance of subgingival gram-negative species was determined. The predominant cultivable microbiota of 9 sites from 3 subjects were isolated immediately before and 6 months after TC fiber treatment. Gram-negative rods were characterized and tested for sensitivity to TC (minimum inhibitory concentration [MIC] 1-128-mu-g/ml), penicillin at 80-mu-g/ml, and erythromycin at 8-mu-g/ml. None of the gram-negative rods were resistant to TC (MIC greater-than-or-equal-to 16-mu-g/ml), either before or after treatment. Before treatment 98% of the gram-negative rods were susceptible to TC at 1-2-mu-g/ml and after therapy 88% were susceptible. The percentage of gram-negative rods exhibiting intermediate TC susceptibility (MIC 4-8-mu-g/ml) changed from 2.0% before therapy to 5.2% after therapy. In no case was either penicillin or erythromycin resistance associated with increased resistance to TC.