Background: Salix 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 Salix that are found naturally in Finland and one cultivated species
were screened for their antimicrobial properties against human pathogenic bacteria. S. starkeana and
S. x pendulina 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 S. myrsinifolia extract showing good
antibacterial effects were identified using UPLC/QTOF-MS. (3) Results: A methanol extract of S. starkeana
was particularly active against B. cereus (MIC 625 μg/mL), and a methanol extract of S. myrsinifolia
showed good activity against S. aureus and B. cereus (MIC 1250 μg/mL) and showed bactericidal effects
during a 24 h incubation of B. cereus. Moreover, a decoction of S. myrsinifolia resulted in good growth
inhibition against P. aeruginosa. Our UPLC/QTOF-MS results indicated that proanthocyanidins (PAs),
and especially the dimer procyanidin B1 (m/z 577) and other procyanidin derivatives, including highly
polymerized proanthocyanidins, were abundant in S. myrsinifolia methanol extracts. Procyanidin B1 and
its monomer catechin, as well as taxifolin and p-hydroxycinnamic acid, all present in S. myrsinifolia twigs,
effectively inhibited B. cereus (MIC 250 μg/mL). (4) Conclusions: This study indicates that Finnish Salix
species contain an abundance of antibacterial condensed tannins, phenolic acids and other polyphenols
that deserve further research for the antibacterial mechanisms of action.
