Efficacy of willow herb (Epilobium angustifolium L. and E. parviflorum Schreb.) crude and purified extracts and oenothein B against prostatic pathogens

Background/Objectives: Plants species of the Epilobium genus are traditionally used to treat prostatitis and other urinary tract disorders and are particularly rich in ellagitannins and flavonol 3-O-glycosides. The aim of this work was to evaluate the inhibitory activity of different extracts from E. angustifolium L. and E. parviflorum Schreb. and their major bioactive compound, oenothein B, against a panel of Gram-positive (Enterococcus faecalis ATCC 29212, Enterococcus faecalis BS, Staphylococcus aureus ATCC 25923, Staphylococcus aureus ATCC 29213, and Staphylococcus aureus ATCC 43300) and Gram-negative (Escherichia coli ATCC 25922, Escherichia coli ATCC 35218, Escherichia coli BS, Klebsiella pneumoniae ATCC 13883, Klebsiella pneumoniae ATCC 70063, Klebsiella pneumoniae BS, Proteus mirabilis BS, and Pseudomonas aeruginosa ATCC 27853) bacteria responsible for prostatitis. Methods: Aqueous and ethanolic raw extracts were prepared, and the latter were further purified using the resin AmberliteTM XAD7HP. Then, an HPLC-MS/MS method was developed for the quantification of the marker bioactives and their levels were correlated with the antimicrobial activity. Results: Purified extracts were richer in polyphenols (330.80 and 367.66 mg/g of dry extract for E. angustifolium and E. parvifolium, respectively) than the raw extracts. Oenothein B was the predominant compound in all the extracts (119.98 to 327.57 mg/g of dry extract). The minimum inhibitory concentrations (MICs) in the range of µg/mL indicated significant antibacterial activity, which was higher for the purified extracts and oenothein B (MIC values from 4 to 16 and 8 to 1024 µg/mL on Gram-positive and Gram-negative strains, respectively). Conclusions: These results outline the outstanding potential of E. angustifolium and E. parviflorum extracts and oenothein B as therapeutic alternatives or complementary agents to conventional antibiotic treatments of prostatitis and other urinary tract infections.