pharmaceutics Review Interactions with Microbial Proteins Driving the Antibacterial Activity of Flavonoids Giuliana Donadio 1,†, Francesca Mensitieri 2,†, Valentina Santoro 1, Valentina Parisi 1,3, Maria Laura Bellone 1,3, Nunziatina De Tommasi 1, Viviana Izzo 2 and Fabrizio Dal Piaz 2,* 1 Department of Pharmacy, University of Salerno, 84084 Fisciano, Italy; [email protected] (G.D.); [email protected] (V.S.); [email protected] (V.P.); [email protected] (M.L.B.); [email protected] (N.D.T.) 2 Department of Medicine and Surgery, University of Salerno, 84082 Baronissi, Italy; [email protected] (F.M.); [email protected] (V.I.) 3 PhD Program in Drug Discovery and Development, Department of Pharmacy, University of Salerno, 84084 Fisciano, Italy * Correspondence: [email protected] † These authors contributed equally to this work. Abstract: Flavonoids are among the most abundant natural bioactive compounds produced by plants. Many different activities have been reported for these secondary metabolites against numerous cells and systems. One of the most interesting is certainly the antimicrobial, which is stimulated through various molecular mechanisms. In fact, flavonoids are effective both in directly damaging the envelope of Gram-negative and Gram-positive bacteria but also by acting toward specific molecular targets essential for the survival of these microorganisms. The purpose of this paper is to present an overview of the most interesting results obtained in the research focused on the study of the Citation: Donadio, G.; Mensitieri, F.; interactions between flavonoids and bacterial proteins. Despite the great structural heterogeneity Santoro, V.; Parisi, V.; Bellone, M.L.; of these plant metabolites, it is interesting to observe that many flavonoids affect the same cellular De Tommasi, N.; Izzo, V.; Dal Piaz, F. pathways. Furthermore, it is evident that some of these compounds interact with more than one Interactions with Microbial Proteins target, producing multiple effects. Taken together, the reported data demonstrate the great potential Driving the Antibacterial Activity of of flavonoids in developing innovative systems, which can help address the increasingly serious Flavonoids. Pharmaceutics 2021, 13, 660. https://doi.org/10.3390/ problem of antibiotic resistance. pharmaceutics13050660 Keywords: flavonoids; antibacterial activity; bioactive natural compounds; enzyme inhibitor; efflux Academic Editor: pumps; ATP synthetase; DNA gyrase; antibiofilm activity Maria Nowakowska Received: 14 April 2021 Accepted: 1 May 2021 1. Introduction Published: 5 May 2021 Natural flavonoids are secondary metabolites widely produced by plants. These are polyphenolic compounds generally characterized by a three-cyclic structure, consisting of Publisher’s Note: MDPI stays neutral two phenyl/benzyl rings (A and B) connected by a heterocycle (C). Depending on the level with regard to jurisdictional claims in of unsaturation and oxidation, and on the position of the different substituents, flavonoids published maps and institutional affil- can be grouped into various subclasses (Figure1): flavones (i.e., apigenin, luteolin, bacalein, iations. tangeritin, diosmetin), in which the B ring is linked to the C ring at position C2; isoflavones (i.e., genistein, daidzein), whose B ring is linked to the C3 position of C ring; flavonols (i.e., myricetin, kaempferol, quercetin) that share a 3-hydroxyflavones backbone; flavanols, which are also named catechins or flavan-3-ols (i.e., epicatechin, epicatechin-3-O-gallate Copyright: © 2021 by the authors. or ECG, epigallocatechin-3-O-gallate or EGCG) and are derived from hydroxylation at Licensee MDPI, Basel, Switzerland. C3 of flavans, benzopyran derivatives sharing the 2-phenyl-3,4-dihydro-2H-chromene This article is an open access article skeleton; flavanones (i.e., naringenin, hesperetin, sakuranetin, pinocembrin), similar to distributed under the terms and flavones, lacking C-C double bonds on the C ring; flavanonols (i.e., dihydrokaempferol, conditions of the Creative Commons dihydroquercetin or taxifolin, silymarin), which are flavanones bearing a hydroxylic group Attribution (CC BY) license (https:// on the C3; anthocyanidins (i.e., cyanidin, delphinidin, pelargonidin, europinidin), which creativecommons.org/licenses/by/ share the flavanols structure, bearing an oxonium ion on oxygen in the C ring; lastly, 4.0/). Pharmaceutics 2021, 13, 660. https://doi.org/10.3390/pharmaceutics13050660 https://www.mdpi.com/journal/pharmaceutics Pharmaceutics 2021, 13, x FOR PEER REVIEW 2 of 23 Pharmaceutics 2021, 13, 660 2 of 23 ing a hydroxylic group on the C3; anthocyanidins (i.e., cyanidin, delphinidin, pelargo- chalconesnidin, europinidin), and dihydrochalcones which share (i.e., the flavanol phloretin,s structure, licochalcone bearing A, B, an C, oxonium D, butein, ion bartericin on oxy- A)gen derive in the from C ring; the lastly, structure chalcones of anthocyanidins and dihydroc athalcones higher pH, (i.e., at phloretin, which the licochalcone C ring opens, A, andB, C, an D, aromatic butein, bartericin ketone is formed.A) derive from the structure of anthocyanidins at higher pH, at which the C ring opens, and an aromatic ketone is formed. FigureFigure 1. 1.Basic Basic structures structures of of the the main main flavonoid flavonoid classes. classes. FlavonoidsFlavonoids are are possibly possibly the the most most abundant abundant secondary secondary metabolites metabolites in in plants, plants, and and their their synthesis is finely regulated, varying with environmental conditions. These compounds synthesis is finely regulated, varying with environmental conditions. These compounds display a range of different physiological roles in vivo, being generlly involved in different display a range of different physiological roles in vivo, being generlly involved in different types of abiotic (visible and ultraviolet light, heat) or biotic (pathogens) stress-induced types of abiotic (visible and ultraviolet light, heat) or biotic (pathogens) stress-induced responses [1,2]. Some flavonoids are produced in a species-specific mode (flavanones responses [1,2]. Some flavonoids are produced in a species-specific mode (flavanones in in citrus fruits, isoflavons in soya, phlorizin in apples); others, such as naringin and citrus fruits, isoflavons in soya, phlorizin in apples); others, such as naringin and querce- quercetin, are ubiquitous in most plants. The huge structural diversity and wide biological tin, are ubiquitous in most plants. The huge structural diversity and wide biological ac- activity of flavonoids becomes evident if all possible backbone modifications are considered. tivity of flavonoids becomes evident if all possible backbone modifications are considered. Moreover, different functional groups are often bound to the polyphenolic structure to fine- Moreover, different functional groups are often bound to the polyphenolic structure to tune their reactivity, stability, solubility, and bioavailability. Glycosylated and methylated fine-tune their reactivity, stability, solubility, and bioavailability. Glycosylated and meth- flavonoids are often produced by the different plant families [3,4]. ylated flavonoids are often produced by the different plant families [3,4]. A role of primary importance for these compounds in plant cells is the defense againstA reactiverole of oxygenprimary species importance (ROS). for The these abundance compounds in hydroxyl in plant groups cells and is conjugatedthe defense doubleagainst bondsreactive in oxygen their chemical species (ROS). structure The make abundance flavonoids in hydroxyl effective groups in ROS and scavenging, conjugated metaldouble chelation, bonds in and their quenching chemical ofstructure lipid peroxidation make flavonoids [5,6]. Anothereffective prominentin ROS scavenging, function ofmetal flavonoids, chelation, correlated and quenching with the of lipid previous peroxi one,dation is the [5,6]. inhibition Another ofprominent auxin transport function in of plantflavonoids, organs. correlated Auxin co-regulates with the previous growth one, and is light-orientation the inhibition of inauxin plants, transport and different in plant studiesorgans. have Auxin highlighted co-regulates that growth the interplay and light-orientation between this hormone in plants, and and flavonoids, different whosestudies productionhave highlighted are both that stimulated the interplay by UV between light, contributes this hormone to the and regulation flavonoids, of the whose UV-induced produc- morphogenesistion are both stimulated [7,8]. Among by UV flavonoids, light, contribu anthocyaninstes to the constituteregulation the of the major UV-induced constituent mor- of plantphogenesis pigments, [7,8]. along Among with flavonoids, chlorophyll anthocya and carotenoids.nins constitute The differential the major enrichmentconstituent inof pelargonidin,plant pigments, cyanin, along or with delphinidin-like chlorophyll and anthocyanins carotenoids. determines The differential flower color,enrichment playing in apelargonidin, central role incyanin, pollinator or delphinidin-like attraction and plantanthocyanins reproduction determines [9]. Physiological flower color, rolesplaying of flavonoidsa central role are notin pollinator limited to attraction plant aerial and parts; plant they reproduction are produced [9]. also Physiological in the roots, roles where of theyflavonoids function are as not chemo-attractors limited to plant for aerial symbionts parts; recruitmentthey are produced from soil, also such in the as