Review Dextran: Sources, Structures, and Properties Elsa Díaz-Montes Unidad Professional Interdisciplinaria de Biotecnología, Instituto Politécnico Nacional, Av. Acueducto s/n, Barrio La Laguna Ticoman, Ciudad de México 07340, Mexico; [email protected] Abstract: Dextran is an exopolysaccharide (EPS) synthesized by lactic acid bacteria (LAB) or their enzymes in the presence of sucrose. Dextran is composed of a linear chain of D-glucoses linked by α-(1!6) bonds, with possible branches of D-glucoses linked by α-(1!4), α-(1!3), or α-(1!2) bonds, which can be low (<40 kDa) or high molecular weight (>40 kDa). The characteristics of dextran in terms of molecular weight and branches depend on the producing strain, so there is a great variety in its properties. Dextran has commercial interest because its solubility, viscosity, and thermal and rheological properties allow it to be used in food, pharmaceutical, and research areas. The aim of this review article is to compile the latest research (in the past decade) using LAB to synthesize high or low molecular weight dextran. In addition, studies using modified enzymes to produce dextran with specific structural characteristics (molecular weights and branches) are addressed. On the other hand, special attention is paid to LAB extracted from unconventional sources to expose their capacities as dextran producers and their possible application to compete with the only commercial strain (Leuconostoc mesenteroides NRRL B512). Keywords: lactic acid bacteria; exopolysaccharides; dextran; structure; properties Citation: Díaz-Montes, E. Dextran: Sources, Structures, and Properties. 1. Introduction Polysaccharides 2021, 2, 554–565. Lactic acid bacteria (LAB) are microorganisms that produce lactic acid as the main https://doi.org/10.3390/ or only product of carbohydrate fermentation (heterofermentation or homofermentation, polysaccharides2030033 respectively). The nutritional requirements are complex, since they are based on vita- mins, minerals, fatty acids, amino acids, peptides, and carbohydrates, which are usually Academic Editors: Cédric Delattre, in their natural habitats [1]. LAB have been isolated from dairy foods, meats, cereals, Paolina Lukova and Guillaume Pierre vegetables, soil, water, and vaginal waste. According to their characteristics and taxon- omy, LAB include bacteria belonging to the genera Aerococcus, Alloiococcus, Carnobacterium, Received: 21 May 2021 Dolosigranulum, Enterococcus, Globicatella, Lactococcus, Lactobacillus, Leuconostoc, Oenococcus, Accepted: 28 June 2021 Published: 1 July 2021 Pediococcus, Streptococcus, Vaiscoccus, and Weiscoccus [2]. LAB are considered probiotic bacteria because they can be incorporated into food to improve the consumer0s intestinal Publisher’s Note: MDPI stays neutral microbial balance, and they are also generally recognized as safe (GRAS) because they with regard to jurisdictional claims in are not pathogenic for humans [3]. On the other hand, they are responsible for a great published maps and institutional affil- diversification of flavors and textures of food products, which is why they are mainly iations. used to produce different fermented products such as yogurt, cheese, sourdoughs, pickles, sausages, and soy products [4]. In addition, some LAB can produce extracellular polysac- charides (called exopolysaccharides, EPS) that are repeat units of sugars such as glucose, galactose, and rhamnose, which are secreted during bacterial growth [5]. EPS can be classified into two groups depending on the units that comprise it. Heteropolysaccharides Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. consist of different monosaccharide units, for example, xanthan and gellan. Homopolysac- This article is an open access article charides are composed of repeating units of a single type of monosaccharides (e.g., glucose distributed under the terms and or fructose), for example, glucans and fructans. Levan and inulin are the fructans produced conditions of the Creative Commons by LAB, while the most commonly produced glucans are cellulose, pullulan, curdlan, Attribution (CC BY) license (https:// mutan, alternan, and dextran [6]. These natural polysaccharides have been used as carriers, creativecommons.org/licenses/by/ encapsulants, thickeners, binders, lubricants, and additives in the pharmaceutical and 4.0/). food industries [7]. However, the most important EPS for medical and industrial use is Polysaccharides 2021, 2, 554–565. https://doi.org/10.3390/polysaccharides2030033 https://www.mdpi.com/journal/polysaccharides Polysaccharides 2021, 2, 2 Polysaccharides 2021, 2 555 the most important EPS for medical and industrial use is dextran, which was initially be- lieved to be synthesized only by Leuconostoc mesenteroides, but subsequent research reported dextran,its segregation which by was another initially type believed of LAB to (see be Sectio synthesizedn 2) [8]. only The byliteratureLeuconostoc on the mesenteroides identification, butor characterization subsequent research of dextrans reported produced its segregation by LAB by has another been increasing type of LAB in (seethe past Section decade,2)[ 8 ].as Thecan literaturebe seen in on Figure the identification 1. Therefore, the or characterization aim of this review of is dextrans to show produced the advances by LABthat have has beenbeen increasing made in the in discovery the past decade,and characterizatio as can be seenn of innew Figure dextrans,1. Therefore, their structural the aim character- of this reviewistics (molecular is to show weight, the advances links, and that branches), have been and made a brief in the description discovery of and their characterization possible appli- ofcations new dextrans, in medical, their food, structural and research characteristics areas. In addition, (molecular emphasis weight, is links, placed and on branches), extraction and a brief description of their possible applications in medical, food, and research areas. In sources for dextran-producing bacterial strains. addition, emphasis is placed on extraction sources for dextran-producing bacterial strains. Figure 1. Number of publications related to dextran synthesis by lactic acid bacteria (source: Scopus; keywords: Bacteria, LeuconostocFigure 1. ,NumberWeissella of, Lactobacillus publications, andrelatedStreptococcus to dextran; accessed: synthesis 3 by May lactic 2021). acid bacteria (source: Scopus; keywords: Bacteria, Leuconostoc, Weissella, Lactobacillus, and Streptococcus; accessed: 3 May 2021). 2. Synthesis of Dextran 2. Synthesis of Dextran Dextran is synthesized in a particular way by LAB when exposed to a medium with sucroseDextran as a carbon is synthesized source in [8 ].a particular In some LABway by (e.g., LABLactobacillus when exposed), sucrose to a medium can enter with the su- cellcrose directly as a carbon via the source phosphotransferase [8]. In some LAB (e.g., system Lactobacillus (PTS) and), sucrose metabolize can enter to form the cellD-lactate directly orvia become the phosphotransferase dextran [9]. Bacterial system dextransucrases, (PTS) and metabolize located to extracellularly, form D-lactate are or responsiblebecome dex- fortran hydrolyzing [9]. Bacterial sucrose dextransucrases, in its fructose located and extr glucoseacellularly, monomers, are responsible forming for an hydrolyzing intermediate su- withcrose glucose in its fructose (glycosyl-enzyme) and glucose monomers, to later carry forming out an their intermediate polymerization with glucose and form (glycosyl- dex- tranenzyme) [10], whileto later the carry resulting out their fructose polymerization enters the bacteria and form through dextran PTS [10], to meetwhile its the metabolic resulting demandfructose [enters11], as the shown bacteria in Figure through2. PTS LAB to that meet report its metabolic dextran demand production [11], are as shown mainly in of Figure the genus2. LABLeuconostoc that report, Weisselladextran production, Lactobacillus are, andmainlyStreptococcus of the genus[10 Leuconostoc], which have, Weissella been isolated, Lactoba- fromcillus different, and Streptococcus plant sources [10], which (e.g., have Agave been salmiana isolated and from pummelo) different plant [12,13 sources] and fermented(e.g., Agave productssalmiana (e.g., and ricepummelo) batter, [12,13] cabbage, and idli fermented batter, and products pickles) (e.g., [14– rice17]. batter, However, cabbage, dextran idli can bat- alsoter, beand synthesized pickles) [14–17]. via enzymatic, However, directly dextran using can dextransacarasesalso be synthesized (sucrose: via enzymatic, 1,6-α-D-glucan directly 6-usingα-D-glucosyltransferase, dextransacarases (sucrose: EC 2.4.1.5) 1,6-α [18-D],-glucan which 6- polymerizeα-D-glucosyltransferase, the glucoses of EC the 2.4.1.5) sucrose [18], in dextran,which polymerize as shown atthe the glucoses top of Figureof the 2sucrose. in dextran, as shown at the top of Figure 2. Polysaccharides 2021, 2 556 Polysaccharides 2021, 2, 3 Figure 2. General carbohydrate pathways and dextrandextran synthesis in lactic acid bacteria: Leuconostoc (black),(black), Weissella (orange),(orange), Lactobacillus (blue),(blue), and and StreptococcusStreptococcus (green).(green). Adapted Adapted from from [9–11,19,20]. [9–11,19,20 ]. In industry, industry, dextran is obtained through the culture of Leuconostoc mesenteroides NRRL B512, because because it itis isconsidered considered a bacteria a bacteria genera generallylly recognized recognized as safe as (GRAS) safe (GRAS) and very and stable