Polyphenols and Phenolic Glucosides in Antibacterial Twig Extracts of Naturally Occurring Salix myrsinifolia (Salisb.), S. phylicifolia (L.) and S. starkeana (Willd.) and the Cultivated Hybrid S. x pendulina (Wender.)

(1) Background: species occurring in Finland have not been well studied for their antimicrobial potential, despite their frequent use for lung and stomach problems in traditional medicine. Thus, twig extracts of three species of that are found naturally in Finland and one cultivated species were screened for their antimicrobial properties against human pathogenic bacteria. and were screened for antibacterial effects for the first time. (2) Methods: An agar diffusion and a microplate method were used for the screenings. Time-kill effects were measured using a plate-count and a microplate method. A DPPH-method using a qualitative TLC-analysis was used to detect antioxidant compounds in antimicrobial extracts. Metabolites from a extract showing good antibacterial effects were identified using UPLC/QTOF-MS. (3) Results: A methanol extract of was particularly active against (MIC 625 µg/mL), and a methanol extract of showed good activity against and (MIC 1250 µg/mL) and showed bactericidal effects during a 24 h incubation of . Moreover, a decoction of resulted in good growth inhibition against . Our UPLC/QTOF-MS results indicated that proanthocyanidins (PAs), and especially the dimer procyanidin B1 ( / 577) and other procyanidin derivatives, including highly polymerized proanthocyanidins, were abundant in methanol extracts. Procyanidin B1 and its monomer catechin, as well as taxifolin and p-hydroxycinnamic acid, all present in twigs, effectively inhibited (MIC 250 µg/mL). (4) Conclusions: This study indicates that Finnish species contain an abundance of antibacterial condensed tannins, phenolic acids and other polyphenols that deserve further research for the antibacterial mechanisms of action.