Antimicrobial Activity and Wound-Healing Capacity of Birch, Beech and Larch Bark Extracts

Abstract

Bark is a major by-product of woodworking industries. The contents of several wood species are known to harbor antimicrobial, antiviral, anti-inflammatory and wound-healing capacities. The aim of this work was to identify beneficial properties of Austrian larch, birch and beech bark extracts for their potential usage as additives or active ingredients in dermatological applications. Bacterial agar diffusion assay and resazurin-based broth microdilution assay were used to evaluate anti-bacterial activity. To gain more insight into the cellular response to bark extracts, viability-, scratch-assays and ELISAs were performed. Birch and beech extracts showed strong antimicrobial activities against Gram-positive bacteria, including Cutibacterium acnes, Staphylococcus epidermidis and MRSA. Wound closure was enhanced with birch and beech extracts as compared to controls in the scratch-assays. Whereas beneficial properties of birch bark components have previously been described, the similar effects of beech extracts are novel. The combined positive effect on wound-healing and antimicrobial activity has great potential for the treatment of various skin diseases, including acne in future dermal applications.

Keywords: antimicrobial activity; bark extract; beech; birch; larch; wound healing.

Figure 1

Chromatograms of bark extracts from larch, birch and beech trees. Different substances of DHBA (dihydroxybenzoic acid), HBA (hydroxybenzoic acid), F (unknown flavonoids), V (Vanillin), C (catechin), EA (ellagic acid), FA (ferulic acid), VA (vanillin acid) and GA (gallic acid) were determined.

Figure 2

Resazurin viability assay results of the 4 bacterial strains; results are displayed as fluorescence intensity (560 Ex/610 Em).

Figure 3

Viability test of HaCaT and THP-1 cell lines upon birch and beech extract treatment shown as percentage compared to the untreated control (UT). Experiments were performed in triplicates, n = 3, error bars on column depict SD. HaCaT unstimulated birch and beech: UT vs. 500 µg/mL p **** and 1000 µg/mL p ****; HaCaT stimulated birch: UT vs. 500 µg/mL p *** and 1000 µg/mL p ****; HaCaT stimulated beech: UT vs. 100 µg/mL p *, 500 µg/mL p ** and 1000 µg/mL p ***; THP-1 unstimulated beech: UT vs. 1000 µg/mL p *; THP-1 stimulated beech: UT vs. 1000 µg/mL p ****. Statistical significance: * p < 0.05, ** p < 0.01, **** p < 0.0001.

Figure 4

Pro-inflammatory cytokine expression (IL-8 and IL1beta) in THP-1 and HaCaT cell supernatants after 24 h incubation with extracts. Columns present means of triplicates (n = 3); the error bars show SD. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001.

Figure 5

Wound-healing capacity of birch and beech extract reported as percentage gap closure at 24 and 48 h. The gap was defined as 0% closed at the beginning of the experiment. A fully closed gap resulted in 100% closure. N = 4, error bars present SD. Statistical significance: ** p < 0.01, *** p < 0.001.