Lactic acid bacteria (LAB) are microorganisms that produce lactic acid as the main or only product of carbohydrate fermentation (heterofermentation or homofermentation, respectively). The nutritional requirements are complex, since they are based on vitamins, minerals, fatty acids, amino acids, peptides, and carbohydrates, which are usually in their natural habitats
[1]. LAB have been isolated from dairy foods, meats, cereals, vegetables, soil, water, and vaginal waste. According to their characteristics and taxonomy, LAB include bacteria belonging to the genera
Aerococcus,
Alloiococcus,
Carnobacterium,
Dolosigranulum,
Enterococcus,
Globicatella,
Lactococcus,
Lactobacillus,
Leuconostoc,
Oenococcus,
Pediococcus,
Streptococcus,
Vaiscoccus, and
Weiscoccus [2]. LAB are considered probiotic bacteria because they can be incorporated into food to improve the consumer′s intestinal microbial balance, and they are also generally recognized as safe (GRAS) because they are not pathogenic for humans
[3]. On the other hand, they are responsible for a great diversification of flavors and textures of food products, which is why they are mainly used to produce different fermented products such as yogurt, cheese, sourdoughs, pickles, sausages, and soy products
[4]. In addition, some LAB can produce extracellular polysaccharides (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 consist of different monosaccharide units, for example, xanthan and gellan. Homopolysaccharides are composed of repeating units of a single type of monosaccharides (e.g., glucose or fructose), for example, glucans and fructans. Levan and inulin are the fructans produced by LAB, while the most commonly produced glucans are cellulose, pullulan, curdlan, mutan, alternan, and dextran
[6]. These natural polysaccharides have been used as carriers, encapsulants, thickeners, binders, lubricants, and additives in the pharmaceutical and food industries
[7]. However, the most important EPS for medical and industrial use is dextran, which was initially believed to be synthesized only by
Leuconostoc mesenteroides, but subsequent research reported its segregation by another type of LAB (see
Section 2)
[8]. The literature on the identification or characterization of dextrans produced by LAB has been increasing in the past decade. Therefore, the aim of this review is to show the advances that have been made in the discovery and characterization of new dextrans, their structural characteristics (molecular weight, links, and branches), and a brief description of their possible applications in medical, food, and research areas. In addition, emphasis is placed on extraction sources for dextran-producing bacterial strains.