Biological Activity of Bark Extracts from Northern Red Oak (Quercus rubra L.): An Antioxidant, Antimicrobial and Enzymatic Inhibitory Evaluation

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.

Potential Use of Quercus dalechampii Ten. and Q. frainetto Ten. Barks Extracts as Antimicrobial, Enzyme Inhibitory, Antioxidant and Cytotoxic Agents

“…As could be observed in Table 5 and Table 6 , the lowest MIC, MBC, and MFC values were obtained for the tannin-rich extracts, QDRE M and QFRE M, respectively. In terms of selectivity of the antibacterial potential, our findings are described by the same trend previously reported in our assessment regarding Q. rubra [ 19 ], for which was also observed a higher affinity for Gram-positive than for Gram-negative ones. As well as for Q. rubra , Q. dalechampii and Q. frainetto barks seem to be more active as antibacterial agents instead of antifungals.…”

“…The antimicrobial potential of the extracts was tested against five bacterial and three fungal strains (see Section 2 ) by evaluating minimum inhibitory concentrations (MIC), minimum bactericidal concentration, and minimum fungicidal concentration (MFC) [ 19 , 26 ].…”

“…For MIC measurement, a slightly modified microdilution method was employed [ 19 , 26 ]. First, bacterial suspensions were prepared by mixing 10 µL of 0.5 McFarland inoculum with 9990 µL of Muller–Hinton broth 2×.…”

“…As long as the selected bacterial strains are cited in literature as possessing the ability to produce biofilm, we evaluated the inhibitory potential of the extracts against biofilm formation using a previously developed method which uses RPMI medium (low-nutritional environment) [ 19 ].…”

“…cerris , Q. macranthera subsp. syspirensis and Q. aucheri were confirmed as cytotoxic agents against the Hep-2 cell line [ 18 ], while a potential antidiabetic potential was proven for Q. rubra (an invasive species for European areas) bark extracts through the inhibition of α -glucosidase activity [ 19 ].…”