**1. Introduction**

Bark plays an important role in protecting woody vascular plants, especially through its content in bioactive compounds with an antimicrobial effect [1]. The bark of woody plants is considered to be a by-product of the forestry and wood industry. This can be an important source of bioactive compounds with a high recovery potential. Numerous studies show the importance and value of bark. Moreover, bark extracts can have biological effects, such as antioxidant, antibacterial, anti-inflammatory, anti-tumor, etc., [2–6]. It has been determined that the bio-activity of bark natural extracts is mainly due to their content in phenolic compounds [7,8].

The beech (*Fagus sylvatica* L.) is one of the most widespread woody vascular plants in Europe and particularly in Romania, with a high economic value [9]. Beech wood is mostly used for fire wood or in the wood processing industry. After processing beech wood, a significant amount of bark is obtained.

Some literature data highlights that beech bark can be a rich source of bioactive compounds [10,11]. Furthermore, in a previous study some of the phenolic compounds obtained from beech bark by hot water extraction were identified (vanillic acid, catechin, taxifolin and syringin) [11,12]. Hofmann et al. identified 37 compounds in beech bark extracts, including catechin, epicatechin, quercetin, taxifolin, procyanidins, syringic acid, and coumaric acid [12]. Regarding the biological activity of beech bark extracts, literature information is quite limited. In previous works, the antibacterial and antitumor activity of beech bark aqueous extracts, obtained by classical extraction, was evaluated [11,13]. Beech bark extracts induced a decrease in A375 melanoma cell viability [13] and antimicrobial activity against *Staphylococcus aureus* including methicillin-resistant strains [11]. In another study, Hofmann et al. [10] found that the most efficient antioxidants in beech bark were the (+)-catechin, procyanidin B dimer 2, (−)-epicatechin, and coniferin isomer 2. Consequently, the research directions are oriented towards the identification and isolation of the bioactive compounds and the description of their mechanisms of action at the level of living organisms, finally with possibility of exploitation and functionalization on an industrial scale.

The main objectives of the current study were: (1) to optimize the extraction yield of phenolic compounds from beech bark (BB) based on an original experimental design; (2) the characterization of the phytochemical profile of optimized beech bark extracts (BBE); (3) the evaluation of the biological potential (antioxidant, antibacterial, antifungal, antimutagen, and enzyme inhibitory activity) of the optimized BBE.
