Effects of licorice on Porphyromonas gingivalis growth and biofilm viability

Objectives: Numerous studies have reported the bactericidal and anti-adhesive properties of plant fractions. The aim of this study was to investigate the effects of a licorice (Glycyrrhiza uralensis) extract on growth and biofilm formation by Porphyromonas gingivalis, the major etiological agent of chronic periodontitis. Methods: The effects on growth and biofilm formation were studied using 96-well polystyrene microplates containing serial dilutions of sterile licorice extract prepared in Todd-Hewitt broth medium supplemented with hemin and vitamin K. Following incubation at 37°C for 48 h under anaerobiosis, bacterial growth was monitored by measuring the absorbance at 655 nm while biofilm was quantified by staining with crystal violet after removal of media and free-floating bacteria. P. gingivalis viability was investigated using XTT/menadione assay. Results: At the lowest concentration tested (4.2 µg/ml), the licorice extract still significantly (P<0.05) inhibited biofilm formation by P. gingivalis. However, the extract could not cause the desorption of a pre-formed P. gingivalis biofilm. The licorice extract also significantly (P<0.05) inhibited growth of P. gingivalis down to a concentration of 4.2 µg/ml. Treatment of a pre-formed P. gingivalis biofilm with the licorice extract (down to a concentration of 8 µg/ml) reduced the cell viability by approximately 20%. Conclusion: The licorice extract prepared from G. uralensis may have a beneficial effect for the prevention and treatment of periodontitis by reducing both growth and biofilm viability by P. gingivalis.