## 4 Discussion
With the exception of _E. coli_, the older oak bark extracts exhibited antibacterial activity against all of the tested species. A previous report documented the antimicrobial activity of the oak bark extract on _S. aureus_ and _C. albicans_. Another recent study reported strong antibacterial activities against _P. aeruginosa_, _Mariniluteicoccus flavus_ and _E. coli_. In the current study, antibacterial activities were found against _S. aureus_ MRSA, _K. pneumoniae_ and _P. aeruginosa_. In addition, the antifungal activity against _C. Krusei_ was highlighted. These antimicrobial activities might be attributed to phenolic compounds, such as ellagic acid, epicatechin and catechin, and compounds with strong antimicrobial activities [6; 25]. _Quercus_ species are known for their complex phytochemical composition, which allows for the possibility of using them as sources of bioactive compounds with pharmacological activity for chronic diseases, such as a metabolic syndrome [26; 27]. Regarding the assayed enzyme inhibitory activities, the extracts showed a significant inhibitory activity on \(\alpha\)-glucosidase. The different plant parts of \(Q\). _robur_ were previously shown to have a high \(\alpha\)-glucosidase inhibitory activity, correlated in some cases to tannin content and tannin derivatives [28]. At the same time, the extracts that were obtained from different plant parts of _Quercus_ species showed a strong \(\alpha\)-glucosidase inhibitory activity. Specific examples of the compounds isolated from _Quercus sp._ with \(\alpha\)-glucosidase inhibitory activity include hexagalloyglucose from the galls of _Q. infectoria_[29], tilioside from the leaves of _Q. gilva_[30] and the acorn kernel-derived polyphenols from _Q. variabilis_[31]. Moreover, other studies showed a similar activity of the bark extracts for other species in the _Quercus_ genus, such as _Q. coccifera_[32] and _Q. rubra_[33], which showed IC\({}_{50}\) values similar to acornbose, an approved alpha-glucosidase inhibitor. These findings support for further preclinical and clinical studies where these effects must be validated [32; 33]. In the current study, we have identified a correlation between the TPC values and the IC\({}_{50}\) values for all the extracts, which highlights the idea that phenolic species contribute to bioactivity. The most active sample against \(\alpha\)-glucosidase - the hydroethanolic extract that was obtained using MAE - also showed the highest TPC value. An optimization study regarding _Q. robur_ bark has concluded that MAE acts as an efficient method for recovering higher TPC values, especially when using hydroalcoholic extraction, which is highlighted in the findings of the present study [34]. A similar trend has been observed for MAE and the hydroalcoholic extraction of _Q. cerris_ bark, showing a higher yield of total polyphenols [12]. Phenolic derivatives, such as resveratrol, can act as tyrosinase inhibitors due to their chemical analogy with tyrosine. This class of phytochemical compounds has the potential to be used in diseases characterized by melanin hyperproduction [33; 35]. Previously, certain research studies assessed the tyrosinase inhibitory activity of _Quercus_ species extracts. Examples include the bark of _Q. coccifera_[32], the bark of _Q. rubra_[33] and the galls of _Q. infectoria_[36]. [PERSON] et al. showed that among 30 species of common temperate trees, the methanolic extract of _Q. robur_ bark was the most active against tyrosinase [37]. Nonetheless, the older bark extracts showed modest activity against tyrosinase, much weaker in comparison to kojic acid as a positive control. This might indicate that the quantity of polyphenols in their composition are not enough to achieve sufficient inhibition. From all the types of extraction applied, hydroethanolic extraction mixed with UAE showed the highest anti-tyrosinase activity (with an IC\({}_{50}\) of 79.8 \(\mu\)g/mL), suggesting that this combination would be suitable for obtaining the most active extract from older oak bark.