A petrolether extract of the areal parts of Hypericum perforatum L. was reported to be active against gram-positive bacteria. Hyperforin, a phloroglucin derivative was shown to be the antimicrobial principle. Hyperforin exhibited an excellent effect against methicillin-resistant strains of Staphylococcus aureus with a MIC value of 1.0 microg/ml. A butanol fraction of St. John's Wort revealed anti-Helicobacter pylori activity with MIC values ranging between 15.6 and 31.2 microg/ml. Recently, hydrous solutions of Hypericum perforatum teas were found to be antimicrobially effective against gram-positive bacteria with special activity towards methicillin-restistant strains of Staphylococcus aureus (MIC values: 1.3 to 2.5 mg herb/ml).
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Reichling et al. (2001) studied this question.