Antimicrobial effect of oral care gel containing hinokitiol and 4-isopropyl-3-methylphenol against intraoral pathogenic microorganisms

OBJECTIVE: Deterioration of oral hygiene is closely related to an increase in severity and mortality of corona virus disease-19 (COVID-19), and also contributes to the development of various diseases such as aspiration pneumonia or Alzheimer’s. Oral care is attracting high interest in Japan, which has entered a super-aging society. In this study, we aimed to investigate whether commercially available Hinora(®) (HO), an oral care gel containing hinokitiol and 4-isopropyl-3-methylphenol (IPMP), has biofilm formation inhibitory and antimicrobial activities against various intraoral pathogen microorganisms. METHOD: Candida species, Aggregatibacter actinomycetemcomitans, Staphylococcus aureus, and Pseudomonas aeruginosa were selected during the study period, all of which were analyzed using antimicrobial disc, microorganism turbidity, and crystal violet assays. In addition, the germ tube test using Candida albicans (C. albicans) was performed with a modification of Mackenzie’s method. Images for morphological observation of the germ tubes were acquired using an inverted microscope. For comparison between products, we used Refrecare(®) (RC), which only contains hinokitiol (not containing IPMP). RESULTS: All the intraoral pathogenic microorganisms showed drug susceptibility against undiluted forms of HO and/or RC. In particular, HO was more effective at lower concentrations than RC. In the HO-added group, inhibition circles were observed in all bacteria except P. aeruginosa when added at a concentration of 0.5 g/mL or more. The optical density values at 590 nm (crystal violet) and/or 600 nm (microorganism turbidity) of all the fungi and bacteria were significantly lower when cultured in medium with HO. Inhibition of growth or biofilm formation was observed when HO was added at a concentration of 0.05 g/mL or higher. To investigate the action mechanism of HO, germ tube tests were performed in C. albicans. The results showed that culturing C. albicans in soybean-casein digest broth with HO (0.05 g/mL) significantly suppressed germ tube formation. CONCLUSIONS: These data suggest that oral care gel-containing hinokitiol and IPMP has strong biofilm formation inhibitory activity, as well as antifungal and antimicrobial effects against Candida fungi and multiple intraoral pathogenic microorganisms. Therefore, it may be a promising treatment option for oral infections.