Halitosis affects a large proportion of the population and is, in most cases, caused by the production of volatile
sulfur compounds (VSCs), particularly methyl
mercaptan and
hydrogen sulfide, by specific bacterial species colonizing the oral cavity. In this study, a supercritical extract of Chinese licorice (Glycyrrhiza uralensis), and its major isoflavans,
licoricidin and
licorisoflavan A, were investigated for their effect on growth, VSC production and
protease activity of Porphyromonas gingivalis, Prevotella intermedia and Solobacterium moorei, which have been associated with
halitosis. The effects of licorice extract,
licoricidin, and
licorisoflavan A on VSC production in a saliva model were also tested. We first showed that
licoricidin and
licorisoflavan A, and to a lesser extent the licorice extract, were effective in inhibiting the growth of all three bacterial species, with minimal inhibitory concentrations in the range of 2-80 µg ml(-1). The licorice extract and the two isolates
licoricidin and
licorisoflavan A, were able to dose-dependently reduce VSC production by P. gingivalis, Prev. intermedia, and S. moorei as well as by a human saliva model. Although the extract and isolates did not inhibit the proteolytic activity of bacteria, they blocked the conversion of
cysteine into
hydrogen sulfide by Prev. intermedia. Lastly, the deodorizing effects of the licorice extract,
licoricidin, and
licorisoflavan A were demonstrated, as they can neutralize P. gingivalis-derived VSCs.
Licorisoflavan A (10 µg ml(-1)) was found to be the most effective by reducing VSC levels by 50%. Within the limitations of this study, it can be concluded that a licorice supercritical extract and its major isoflavans (
licoricidin and
licorisoflavan A) represent natural ingredients with a potential for reducing bacterial VSC production and therefore for controlling
halitosis.