We report the inhibition of the causative agents of dental caries, Streptococcus mutans and other oral streptococci, by the antimicrobially active ingredients of the hop plant (Humulus lupulus L.). The hop constituents studied were purified beta acid, xanthohumol, iso-alpha acid and tetra iso-alpha acid. Cruder hop extracts were also investigated. The antimicrobial activity of these hop constituents was tested against four strains of Streptococcus mutans as well as one strain each of Streptococcus sanguis and Streptococcus salivarius and compared to antimicrobial essential oils used in mouthwashes in two independent assay systems. We found that all tested hop constituents inhibited the Streptococci. The minimum inhibitory concentration at pH 7.5 ranged from 2 to 50 µg/ml depending on the microorganism and hop phytochemical tested. Contrary to a previous report, there was no activity enhancement by ascorbic acid over and above the enhancement due to pH lowering. There was no resistance development to beta acid after 10 passages in a subinhibitory concentration of this acid. Antimicrobial activity of hop constituents was found to be greater than other plant products such as thymol, nerol, cinnamon oil, oil of clove, menthol and eucalyptol. The possibilities of using hop constituents in mouthwashes are discussed.
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Vol. 57 • No. 1