The production of homopolysaccharides (dextrans, mutans) and heteropolysaccharides by lactic acid bacteria, their chemical composition, their structure and their synthesis are outlined. Mutans streptococci, which include Streptococcus mutans and S. sobrinus produce soluble and insoluble α‐glucans. The latter may contain as much as 90%α‐1–3 linkages and possess a marked ability to promote adherence to the smooth tooth surface causing dental plaque. Dextrans produced by Leuconostoc mesenteroides are high molecular weight α‐glucans having 1–6, 1–4 and 1–3 linkages, varying from slightly to highly branched; 1–6 linkages are predominant. Emphasis is put on exopolysaccharide producing thermophilic and mesophilic lactic acid bacteria, which are important in the dairy industry. The produced polymers play a key role in the rheological behaviour and the texture of fermented milks. One of the main problems in this field is the transitory nature of the thickening trait. This instability is not yet completely understood. Controversial results exist on the sugar composition of the slime produced, but galactose and glucose have always been identified with galactose predominating in most cases.

Summary -The exopolysaccharide (EPS)-producing gram-positive bacteria which have been studied extensively in the last 10 years are the dairy lactic acid bacteria used in the manufacture of fermented milks such as Streptococcus thermophilus, Lactobacillus delbrueckii subsp bulgaricus and Lactococcus lactis subsp cremoris. The role of exopolysaccharides in the manufacture of fermented milks and in particular yoghurt is weil established; that is, they are essential for proper consistency and texture. A number of strains of these bacteria are capable of producing heteropolysaccharides composed of Iinear or branched repeating units varying in size from disaccharides to heptasaccharides.The final exopolysaccharide of high molecular weight (1 to 2 x 106) is formed by polymerisation of some hundreds to several thousands of these repeating units. Dairy propionibacteria are also capable of producing exopolysaccharides, but this area of research has received comparatively Iittle attention. Nevertheless, similarities appear when the few results obtained with propionibacteria are compared with those concerning dairy lactic acid bacteria. Fermentation conditions (temperature and incubation time) and medium composition (carbon and nitrogen sources) affect the polymer yield and the sugar composition of the polymer. The most frequent identified monosaccharides in polysaccharides formed by propionibacteria are glucose, galactose and mannose. Small amounts of fucose and rhamnose have also been found. A striking difference exists in the molecular weight of polysaccharides produced by lactic acid bacteria and propionibacteria (le, values for exopolysaccharides from the latter are in the range of 200 to 5 000). The polymer-producing ability is an extremely unstable property; it seems to be linked to the presence of plasmids of varying size in mesophilic lactic acid bacteria, whereas most of the exopolysaccharide-producing strains of thermophilic lactic acid bacteria do not harbour plasmids. Propionibacteria harbour plasmids, but their functions are not clearly established; therefore, the question of whether the EPS-producing trait of propionibacteria is plasmid encoded cannot be answered yet.

We developed a chemically defined medium (CDM) containing lactose or glucose as the carbon source that supports growth and exopolysaccharide (EPS) production of two strains of Lactobacillus delbrueckii subsp. bulgaricus. The factors found to affect EPS production in this medium were oxygen, pH, temperature, and medium constituents, such as orotic acid and the carbon source. EPS production was greatest during the stationary phase. Composition analysis of EPS isolated at different growth phases and produced under different fermentation conditions (varying carbon source or pH) revealed that the component sugars were the same. The EPS from strain L. delbrueckii subsp. bulgaricus CNRZ 1187 contained galactose and glucose, and that of strain L. delbrueckii subsp. bulgaricus CNRZ 416 contained galactose, glucose, and rhamnose. However, the relative proportions of the individual monosaccharides differed, suggesting that repeating unit structures can vary according to specific medium alterations. Under pH-controlled fermentation conditions, L. delbrueckii subsp. bulgaricus strains produced as much EPS in the CDM as in milk. Furthermore, the relative proportions of individual monosaccharides of EPS produced in pH-controlled CDM or in milk were very similar. The CDM we developed may be a useful model and an alternative to milk in studies of EPS production.Production of exopolysaccharides (EPS) by lactic acid bacteria in milk is an important factor in assuring the proper consistency and texture of fermented food (14). These heteropolysaccharides are composed of linear and branched repeating units varying in size from tetra-to heptasaccharides. The final EPS of high molecular mass (1 ϫ 10 6 to 2 ϫ 10 6 Da) is formed by polymerization of several hundred to a few thousand of these repeating units.Using milk as a fermentation medium, EPS yields range from 50 to 425 mg/liter (2,3,4,10,16). Although milk medium is relevant to the food industry, EPS isolation from such a complex medium is tedious and time-consuming. Furthermore, EPS purification is hindered by glycohydrolases present in the crude preparations that are capable of degrading EPS. MRS, the usual medium for laboratory fermentation using Lactobacillus delbrueckii subsp. bulgaricus, contains compounds (e.g., beef extract, peptone, yeast extract) that interfere with the analysis of EPS (12). Semidefined medium developed by Kimmel and Roberts (12) and chemically defined medium (CDM) developed by Grobben et al. (11) circumvent the problem of interference, but the ability of these media to support growth and EPS production of various strains of L. delbrueckii subsp. bulgaricus has not been evaluated, and no comparison between EPS produced in milk and in the defined medium is available.In this work, we developed a CDM for two strains of L. delbrueckii subsp. bulgaricus for which EPS production and composition in milk have been previously studied. We examined the influence of medium constituents and fermentation parameters on growth, EPS yield, and sugar composition. Our resu...

Exopolysaccharide production by LactobaciUlus casei CGll was studied in basal minimum medium containing various carbon sources (galactose, glucose, lactose, sucrose, maltose, melibiose) at concentrations of 2, 5, 10, and 20 g/liter. L. casei CGll produced exopolysaccharides in basal minimum medium containing each of the sugars tested; lactose and galactose were the poorest carbon sources, and glucose was by far the most efficient carbon source. Sugar concentrations had a marked eflect on polymer yield. Plasmid-cured Mucderivatives grew better in the presence of glucose and attained slightly higher populations than the wild-type strain. The values obtained with lactose were considerably lower for both growth and exopolysaccharide yield. The level of specific polymer production per cell obtained with glucose was distinctively lower for Mucderivatives than for the Muc+ strain. The polymer produced by L. casei CGll in the presence of glucose was different from that formed in the presence of lactose. The polysaccharide produced by L. casei CGII in basal minimum medium containing 20 g of glucose per liter had an intrinsic viscosity of 1.13 dl/g. It was rich in glucose (76%), which was present mostly as 2or 3-linked residues along with some 2,3 doubly substituted glucose units, and in rhamnose (21%), which was present as 2-linked or terminal rhamnose; traces of mannose and galactose were also present.

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