**2.1. Homo-EPS**

Some LAB can produce EPS that are either secreted to the environment or attached to the cell surface forming capsules. EPS are classified into two groups: homo-EPS, consisting of a single type of monosaccharide (α-D-glucans, β-D-glucans, fructans, and others represented by polygalactan) and hetero-EPS, composed of different types of monosaccharides, mainly D-glucose, D-galactose, L-rhamnose, and their derivatives [16].

The differences arise between the homopolysaccharides mainly because of the features of their primary structure such as the pattern of main chain bonds, molecular weight, and branch structure. Two important groups of homo-EPS are produced by LAB; (i) α-glucans, mainly composed of α-1,6- and α-1,3-linked glucose residues, namely dextrans, produced by *Leuconostoc mesenteroides* subsp. *mesenteroide*s and *Leuconostoc mesenteroides* subsp. *dextranicum* and mutans produced by *Streptococcus mutans* and *Streptococcus sobrinus*; and (ii) fructans, mainly composed of β-2,6-linked fructose molecules, such as levan produced by *Streptococcus salivarius* [17].

The formation of dextran from sucrose has been recorded for *Leuc. mesenteroides* subsp. *mesenteroides*. However, the ability to form dextran is often lost when serial transfers are made in media with increasing salt concentrations. Nevertheless, non-dextran-producing strains of *Leuconostoc* sp. can revert to dextran production when they are inoculated into medium containing tomato or orange juice [18]. In the 1950s, the use of a cell-free enzyme solution permitted dextran synthesis under controlled conditions yielding a polymer of greater purity. A common feature of all dextrans is the preponderance of α-1,6-linkages with branch points at positions 2, 3, or 4 [17]. Some strains of *Leuconostoc amelibiosum* [19] and *Lactobacillus curvatus* [20] are reported to be dextran-producing strains.

Mutan is the glucan synthesized by various serotypes of *Str. mutans*, and differs from dextran in that it contains a high percentage of α-1,3 linkages. Differences in solubility result

β-2,1 (β-2,6)

**Table 1.** Homo EPS produced by LAB

516 Lactic Acid Bacteria – R & D for Food, Health and Livestock Purposes

genetics, and bioactivities of the EPS produced by LAB.

D-glucose, D-galactose, L-rhamnose, and their derivatives [16].

**2. Chemical composition of EPS** 

improving host health" [14].

**2.1. Homo-EPS** 

*Streptococcus salivarius* [17].

strains.

ingredients that exert a beneficial effect on the health of the host. Beneficial microorganisms in the intestine are enhanced by "prebiotics," which are defined as "nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and activity of one or a limited number of bacterial species already resident in the colon, and thus

Most of the current prebiotics are low molecular weight except for inulin. As long carbohydrate chains are metabolized more slowly than the short ones, and polysaccharides thus exert prebiotic effects in more distal colonic regions compared to oligosaccharides, which are more rapidly digested in the proximal colon [15]. Therefore, EPS produced by LAB can be used as prebiotics. This chapter reviews the physicochemical properties,

Some LAB can produce EPS that are either secreted to the environment or attached to the cell surface forming capsules. EPS are classified into two groups: homo-EPS, consisting of a single type of monosaccharide (α-D-glucans, β-D-glucans, fructans, and others represented by polygalactan) and hetero-EPS, composed of different types of monosaccharides, mainly

The differences arise between the homopolysaccharides mainly because of the features of their primary structure such as the pattern of main chain bonds, molecular weight, and branch structure. Two important groups of homo-EPS are produced by LAB; (i) α-glucans, mainly composed of α-1,6- and α-1,3-linked glucose residues, namely dextrans, produced by *Leuconostoc mesenteroides* subsp. *mesenteroide*s and *Leuconostoc mesenteroides* subsp. *dextranicum* and mutans produced by *Streptococcus mutans* and *Streptococcus sobrinus*; and (ii) fructans, mainly composed of β-2,6-linked fructose molecules, such as levan produced by

The formation of dextran from sucrose has been recorded for *Leuc. mesenteroides* subsp. *mesenteroides*. However, the ability to form dextran is often lost when serial transfers are made in media with increasing salt concentrations. Nevertheless, non-dextran-producing strains of *Leuconostoc* sp. can revert to dextran production when they are inoculated into medium containing tomato or orange juice [18]. In the 1950s, the use of a cell-free enzyme solution permitted dextran synthesis under controlled conditions yielding a polymer of greater purity. A common feature of all dextrans is the preponderance of α-1,6-linkages with branch points at positions 2, 3, or 4 [17]. Some strains of *Leuconostoc amelibiosum* [19] and *Lactobacillus curvatus* [20] are reported to be dextran-producing

Mutan is the glucan synthesized by various serotypes of *Str. mutans*, and differs from dextran in that it contains a high percentage of α-1,3 linkages. Differences in solubility result

from the proportions of different types of linkages; water-soluble glucans are rich in α-1,6 linkages, while water-insoluble glucans are rich in α-1,3 linkages [17]. Ingestion of mutan has been linked with dental caries, as insoluble mutans can adhere to teeth, thus helping microorganisms adhere to the surface of teeth.

Exopolysaccharides of Lactic Acid Bacteria for Food and Colon Health Applications 519

LAB strain culture would be a useful method to produce EPS for food applications if the LAB could be grown in edible and safe culture media such as whey, and if fermentation

Fermentation conditions using undefined media have been improved to maximize yields. However, a chemically defined medium containing a carbohydrate source, mineral salts, amino acids, vitamins, and nucleic acid bases is more suitable for investigating the influence of different nutrients on LAB growth and EPS biosynthesis. The total yield of EPS produced by LAB depends on the composition of the medium (carbon and nitrogen sources) and the

Under conditions of higher temperatures and slower growth, the production of the polymer per cell in *Lb. delbrueckii* subsp. *bulgaricus* NCFB 2772 was greater in milk [57]. Another study investigated the optimum culture conditions for EPS production by *Lb. delbrueckii* subsp. *bulgaricus* RR in semidefined medium [58], and determined the optimum temperature and pH conditions for EPS production to be 36°C - 39°C and pH 4.5 - 5.5. The optimal temperature for EPS production was approximately 40°C for thermophilic LAB strains, and around 25°C for mesophilic LAB. Gamar et al. [59] reported increased slime production at lower incubation temperatures, and an increase in the final EPS concentration in *Lb. rhamnosus* following incubation at 25°C instead of 30°C. The effects of temperature on EPS production in whey were investigated in *Lb. plantarum* [53], and the yield was found to be higher at 25°C than at either 30°C or 37°C. Moreover, an inverse relationship was observed between EPS production per cell and the growth temperature for *Lactobacillus sake* [49], i.e. the lower the temperature, the higher the EPS production per cell. However, the growth rate in the exponential phase decreased at low temperatures. Therefore, the temperature for the maximal production of EPS is based on a balance of cell density and EPS production per cell. Maximal EPS production by *Lb. sake* was obtained under anaerobic conditions at 20°C, although EPS production per cell was higher at 10°C. Therefore, it is possible that severe environmental conditions trigger EPS production as a protective

The effects of alterations to the nitrogen and carbon sources used in EPS production have also been investigated. According to early reports, neither LAB growth nor EPS production was specifically linked to the presence of casein or whey proteins in the growth medium. Garcia et al. [57] reported that EPS production by *Lb. delbrueckii* subsp. *bulgaricus* NCFB 2772 increased during the early growth pase in the presence of hydrolyzed casein in milk, while the addition of hydrolyzed casein to MRS medium did not increase EPS production. This strain produced 25 mg/l EPS when grown on fructose in a defined medium, and 80 mg/l EPS when grown on glucose [60]. The optimum Bacto-casitone concentration for EPS production by *Lb. delbrueckii* subsp. *bulgaricus* RR was investigated in semidefined medium [58]. In this study, there was a significant relationship between the Bacto-casitone concentration and EPS production; the higher the casitone concentration, the higher the EPS yield that was obtained. For *Lb. plantarum* grown in whey, yeast extract was a more effective nitrogen source for EPS production than soy peptide, tryptone, peptone, and Lab-Lemco powder,

conditions were optimized to obtain a high yield.

mechanism.

growth conditions, i.e., temperature, pH, and incubation time.

Alternan has alternate α-1,6 and α-1,3 linkages, and this structure is thought to be responsible for its distinctive physical properties including high solubility and low viscosity. These characteristics provide this glucan with a potential commercial application as a low viscosity texturizer in foods. *Leuc. mesenteroides* NRRL B-1355 was first reported to be an alternan-producing strain [21]

Levan is an EPS produced from sucrose. It is fructan composed of β-2,6-linked fructose molecules with some β-2,1-linked branches. Incidentally, inulin is a fructan composed of β-2,1-linked fructose molecules with some β-2,6-linked branches. *Str. salivarius, Leuc. mesenteroides,* and *Lactobacillus reuteri* are known to be levan-producing LAB [22-23]. In addition, the EPS produced by *Lactobacillus sanfranciscensis* TMW 1.392 has been reported to be fructan [11].
