Combined effects of nitrate and antimicrobial compounds on in vitro subgingival biofilms

Chlorhexidine and antibiotics are commonly used as adjunct treatments for periodontitis. However, these antimicrobials can lead to microbial resistance an impair health-associated nitrate (NO3-) metabolism. We tested the effect of chlorhexidine (0.002%), metronidazole (0.002% ) and amoxicillin (0.002% ) on the bacterial composition and NO3- metabolism of in vitro-grown subgingival plaque samples from 12 periodontal patients. The low concentrations of amoxicillin and chlorhexidine significantly inhibited microbial growth and impaired NO3- reduction, whereas the physiological concentration of metronidazole did not. A lower SMDI dysbiosis index was found in the NO3- and in the metronidazole + NO3- conditions compared with amoxicillin alone and chlorhexidine alone (p < 0.05). Moreover, NO3- alone or combined with metronidazole appeared to increase Neisseria spp. and Aggregatibacter spp., whilst disease-associated species increased in the chlorhexidine condition. A dual treatment of metronidazole + NO3- should be further investigated in clinical studies.