Abstract
Enterococcus faecalis is one of the bacterial species most frequently isolated from persistent endodontic and apical periodontal infections. The aim of the present study was to evaluate the synergistic antibacterial effects of nisin and selected licorice polyphenols (glabridin, licoricidin, licochalcone A) against planktonic and biofilm-embedded E. faecalis cells. The biocompatibility and anti-inflammatory properties of the nisin/licorice polyphenol combinations were also investigated. The lantibiotic bacteriocin (nisin), the two isoflavonoids (glabridin, licoricidin), and the chalcone (licochalcone A) efficiently inhibited the growth of E. faecalis, with MICs ranging from 6.25 to 25 µg/mL. Combining nisin with each licorice polyphenol individually resulted in a significant synergistic antibacterial effect. Following a 30-min contact, nisin in combination with either glabridin, licoricidin, or licochalcone A caused significant biofilm killing. The nisin/licorice polyphenol combinations had no cytotoxic effects (oral epithelial cells, gingival fibroblasts, and stem cells of the apical papilla), with the exception of nisin/glabridin, when used at their MICs. Lastly, we showed that nisin/glabridin, nisin/licoricidin, and nisin/licochalcone A inhibit NF-κB activation induced by E. faecalis in a monocyte model, suggesting that these combinations possess anti-inflammatory properties. The present study provides evidence that combinations of nisin and glabridin, licoricidin, or licochalcone A show promise as root canal disinfection agents.
Highlights
Many microorganisms colonizing the oral cavity can penetrate and infect the dental pulp, only a limited number of species are truly capable of doing so given the particular environment of the root canal (Siqueira and Rocas 2009)
Given the synergistic antibacterial interactions observed with the combinations of nisin and the licorice polyphenols, the second part of the present study investigated nisin/glabridin, nisin/licoricidin, and nisin/licochalcone A, which were tested for their bactericidal effects on pre-formed E. faecalis biofilms
As the biocompatibility of antimicrobial agents used for root canal disinfection is an important aspect that must be taken into consideration, we investigated the cytotoxic effects of nisin/glabridin, nisin/licoricidin, and nisin/licochalcone A on three human cell types: oral epithelial cells, gingival fibroblasts, and stem cells of the apical papilla
Summary
Many microorganisms colonizing the oral cavity can penetrate and infect the dental pulp, only a limited number of species are truly capable of doing so given the particular environment of the root canal (Siqueira and Rocas 2009). Once they gain access to the complex root canal system, bacteria adhere to the dentine surface and form a biofilm, which is a complex aggregation of microorganisms entrapped in a polymer network mainly composed of polysaccharides and DNA. When the NF-kB signaling pathway is activated by bacteria or their cell surface components such as lipopolysaccharide (LPS) and lipoteichoic acid, these cells release pro-inflammatory mediators that contribute to tissue destruction (Gomes and Herrera 2018)
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