Antimicrobial Effects of Inula viscosa Extract on the In Situ Initial Oral Biofilm

Given the undesirable side effects of commercially used mouth rinses that include chemically synthesized antimicrobial compounds such as chlorhexidine, it is essential to discover novel antimicrobial substances based on plant extracts. The aim of this study was to examine the antimicrobial effect of Inula viscosa extract on the initial microbial adhesion in the oral cavity. Individual test splints were manufactured for the participants, on which disinfected bovine enamel samples were attached. After the initial microbial adhesion, the biofilm-covered oral samples were removed and treated with different concentrations (10, 20, and 30 mg/mL) of an I. viscosa extract for 10 min. Positive and negative controls were also sampled. Regarding the microbiological parameters, the colony-forming units (CFU) and vitality testing (live/dead staining) were examined in combination with fluorescence microscopy. An I. viscosa extract with a concentration of 30 mg/mL killed the bacteria of the initial adhesion at a rate of 99.99% (log(10) CFU value of 1.837 ± 1.54). Compared to the negative control, no killing effects were determined after treatment with I. viscosa extract at concentrations of 10 mg/mL (log(10) CFU value 3.776 ± 0.831; median 3.776) and 20 mg/mL (log(10) CFU value 3.725 ± 0.300; median 3.711). The live/dead staining revealed a significant reduction (p < 0.0001) of vital adherent bacteria after treatment with 10 mg/mL of I. viscosa extract. After treatment with an I. viscosa extract with a concentration of 30 mg/mL, no vital bacteria could be detected. For the first time, significant antimicrobial effects on the initial microbial adhesion in in situ oral biofilms were reported for an I. viscosa extract.