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In the search for alternative treatment options for infections with multi-resistant germs, traditionally used medicinal plants are currently being examined more intensively. In this study, the antimicrobial and anti-biofilm activities of 14 herbal drugs were investigated. Nine of the tested drugs were traditionally used in Europe for treatment of local infections. For comparison, another five drugs monographed in the European Pharmacopoeia were used. Additionally, the total tannin and flavonoid contents of all tested drugs were analyzed. HPLC fingerprints were recorded to obtain further insights into the components of the extracts. The aim of the study was to identify herbal drugs that might be useable for treatment of infectious diseases, even with multidrug resistant E. coli, and to correlate the antimicrobial activity with the total content of tannins and flavonoids. The agar diffusion test and anti-biofilm assay were used to evaluate the antimicrobial potential of different extracts from the plants. Colorimetric methods (from European Pharmacopeia) were used for determination of total tannins and flavonoids. The direct antimicrobial activity of most of the tested extracts was low to moderate. The anti-biofilm activity was found to be down to 10 µg mL−1 for some extracts. Tannin contents between 2.2% and 10.4% of dry weight and total flavonoid contents between 0.1% and 1.6% were found. Correlation analysis indicates that the antimicrobial and the anti-biofilm activity is significantly (p < 0.05) dependent on tannin content, but not on flavonoid content. The data analysis revealed that tannin-rich herbal drugs inhibit pathogens in different ways. Thus, some of the tested herbal drugs might be useable for local infections with multi-resistant biofilm-forming pathogens. For some of the tested drugs, this is the first report about anti-biofilm activity, as well as total tannin and flavonoid content.
Species of the genus Drosera, known for carnivorous plants, such as sundew, have been traditionally used for centuries as medicinal plants. Efficacy-determining compounds are naphthoquinones and flavonoids. Flavonoids possess a broad spectrum of bioactive properties, including biofilm inhibitory activity. Biofilms render antibiotics ineffective, contributing to the current rise in antimicrobial resistance. In this study, the biofilm inhibitory activity of two European sundew species (Drosera rotundifolia and Drosera intermedia) grown agriculturally in Germany and four commercial sundew products (declared as Drosera longifolia, Drosera sp. and Drosera planta trit.) against three multidrug-resistant Escherichia coli strains was tested. The aim of the study was to comparatively investigate the biofilm inhibitory potential of sundew species extracts grown locally in northern Germany and commercial sundew products. The minimum biofilm inhibitory concentration of the European sundew species was approx. 35 µg mL−1. In comparison, commercial sundew products ranged in concentration from 75 to 140 µg mL−1. Additionally, individual compounds isolated from European sundew were tested. Among these compounds, biofilm inhibitory activity was determined for four of the eight substances, with 2″-O-galloyl hyperoside standing out for its activity (38 µg mL−1). The whole plant extracts of Drosera rotundifolia and Drosera intermedia proved to be more effective than the commercial products and the single compounds in its biofilm inhibition activity against Escherichia coli strains. Sundew extracts may serve as a potential therapeutic approach for targeting biofilm production.