Citation: Tanase, C.; Nicolescu, A.; Nisca, A.; S , tef˘ anescu, R.; Babot ˘ a, M.; Mare, A.D.; Ciurea, C.N.; Man, A. Biological Activity of Bark Extracts from Northern Red Oak (Quercus rubra L.): An Antioxidant, Antimicrobial and Enzymatic Inhibitory Evaluation. Plants 2022, 11, 2357. https://doi.org/10.3390/ plants11182357 Academic Editor: Maria José U. Ferreira Received: 4 August 2022 Accepted: 7 September 2022 Published: 9 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Biological Activity of Bark Extracts from Northern Red Oak (Quercus rubra L.): An Antioxidant, Antimicrobial and Enzymatic Inhibitory Evaluation Corneliu Tanase 1,2,† , Alexandru Nicolescu 3,† , Adrian Nisca 4 , Ruxandra S , tefănescu 5, * , Mihai Babotă 3 , Anca Delia Mare 6 , Cristina Nicoleta Ciurea 6 and Adrian Man 6 1 Department of Pharmaceutical Botany, Faculty of Pharmacy, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, 38 Gheorghe Marinescu Street, 540139 Targu Mures, Romania 2 Research Center for Medicinal and Aromatic Plants, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, 38 Gheorghe Marinescu Street, 540139 Targu Mures, Romania 3 Faculty of Pharmacy, “Iuliu Hat , ieganu” University of Medicine and Pharmacy, 8 Victor Babes Street, 400012 Cluj-Napoca, Romania 4 Doctoral School of Medicine and Pharmacy, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, 38 Gheorghe Marinescu Street, 540139 Targu Mures, Romania 5 Department of Pharmacognosy and Phytotherapy, Faculty of Pharmacy, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, 38 Gheorghe Marinescu Street, 540139 Targu Mures, Romania 6 Department of Microbiology, Faculty of Medicine, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, 38 Gheorghe Marinescu Street, 540139 Targu Mures, Romania * Correspondence: ruxandra.stefanescu@umfst.ro † These authors contributed equally to this work. Abstract: The northern red oak (Quercus rubra L.) is an ornamental oak species native to eastern America, being an invasive species in Europe, with increasing coverage. The aim of this work was to evaluate the biological potential of red oak bark extracts. Aqueous and ethanolic preparations were obtained by two extraction methods: ultrasonic-assisted extraction (UAE) and microwave assisted extraction (MAE). The total phenolic and tannin contents were measured using spectrophotometric methods. The antioxidant activity was evaluated by two complementary methods (DPPH and ABTS). Antimicrobial potential was tested against five bacteria and three Candida species, and the effect on biofilm formation and synergism with gentamicin was also evaluated. Finally, enzyme inhibitory properties were assessed for α-glucosidase, tyrosinase, and acetylcholinesterase. The results indicated a higher phenolic content for the extracts obtained through MAE, while UAE bark extracts were rich in tannins. All the extracts exhibited antioxidant, anti-glucosidase, and anti-tyrosinase activity, while the antibacterial potential was mostly observed for the MAE extracts, especially against S. aureus, C. parapsilopsis, and C. krusei; inhibition of biofilm formation was observed only for MRSA. These findings show that the red oak bark might be an important source of bioactive compounds with antioxidant and antimicrobial properties. Keywords: antibacterial; antidiabetic; antifungal; antioxidant; bark; polyphenols; red oak; Quercus; tannins 1. Introduction The northern red oak (Quercus rubra L. syn. Q. borealis F. Michx.) is an ornamental oak species belonging to the Fagaceae family, being native to eastern America [1]. It was introduced to Europe in 1961 and currently covers wide areas in northern hemisphere [1,2], where is cultivated for wood; unfortunately, it has lower quality and durability in compari- son to native oaks (Q. robur, Q. petraea)[3]. Plants 2022, 11, 2357. https://doi.org/10.3390/plants11182357 https://www.mdpi.com/journal/plants