Validation of antibiotic efficacy on in vitro subgingival biofilms

Systemic antibiotics are often used as adjunctive treatment modalities for periodontal diseases. Testing of antimicrobial efficacy can be relevant only if the bacteria are in the form of biofilms rather than the planktonic state, and at concentrations of physiologic relevance, i.e., reachable in the periodontal pocket. The aim of the present study is to test the antimicrobial efficacy of five common antibiotic schemes at physiologically relevant concentrations on a multispecies in vitro biofilm model.

A 10-species in vitro subgingival biofilm model was exposed to metronidazole (15 μg/mL), amoxicillin (15 μg/mL), metronidazole and amoxicillin in combination, doxycycline (2 μg/mL), and azithromycin (10 μg/mL) over 24 hours. Species-specific bacterial numbers were determined by culture on selective agar media or by epifluorescence microscopy.

Metronidazole alone did not affect biofilm composition. Total bacterial counts were significantly reduced by doxycycline, azithromycin, and amoxicillin alone or in combination with metronidazole, albeit by less than 1 log. On the species-specific level, these regimens significantly reduced the numbers of Streptococcus anginosus, Porphyromonas gingivalis, and Fusobacterium nucleatum, as well as Campylobacter rectus (except for amoxicillin alone). The strongest effects were displayed by the combination of amoxicillin and metronidazole.

Antibiotics at concentrations detectable in gingival crevicular fluid do not dramatically reduce total bacterial loads in this in vitro biofilm model, but cause species-specific reductions, which may disrupt the biofilm unity.