Antibiotic Resistance in Periodontal Streptococcus intermedius Clinical Isolates

Objectives: Streptococcus intermedius is a prominent subgingival isolate in some refractory periodontitis patients, and may resist mechanical forms of periodontal therapy. Since adjunctive antimicrobial therapy may thus be indicated for S. intermedius periodontal infections, the occurrence of in vitro antibiotic resistance to doxycycline, amoxicillin, metronidazole and clindamycin was determined for fresh S. intermedius subgingival isolates from 300 consecutive organism-positive severe periodontitis subjects. Methods: Pooled subgingival plaque specimens from each study subject were transported in VMGA III, and plated onto non-selective enriched Brucella blood agar (EBBA), and onto EBBA supplemented with either 4 µg/ml of doxycycline or clindamycin, 8 µg/ml of amoxicillin, or 16 µg/ml of metronidazole (representing CLSI or Norwegian Antibiotic Working Group non-susceptible breakpoint concentrations), followed by anaerobic incubation. S. intermedius isolates were identified as gram-positive, lactose MUG-test positive, non-motile, facultative cocci exhibiting small dry, white, raised colonies with wrinkled edges on anaerobically incubated EBBA. In vitro antibiotic resistance was noted as S. intermedius growth on both antibiotic-supplemented and non-supplemented EBBA. Results: S. intermedius recovery averaged 5.9% (range 0.1-67.0%) of total cultivable anaerobic counts on non-supplemented EBBA among all study subjects. A total of 168 (56%) subjects yielded S. intermedius resistant to 4 µg/ml doxycycline, and 184 (61.3%) with S. intermedius resistant to 16 µg/ml metronidazole. S. intermedius resistance to 4 µg/ml clindamycin, or to 8 µg/ml amoxicillin, was found in only 33 (11%) and 3 (1%) study subjects, respectively. Conclusions: These findings indicate that in periodontitis patients colonized by subgingival S. intermedius, caution should be employed with periodontal therapies employing doxycycline or metronidazole, due to the relatively high frequency of clinical strains demonstrating a lack of susceptibility to those antibiotic medications. Considerably greater clinical benefit may be expected with adjunctive use of amoxicillin-based therapies, as well as often with clindamycin, in treatment of S. intermedius periodontal infections.