Production of exopolysaccharides by lactic acid bacteria and dairy propionibacteria

Abstract

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 well 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 linear or branched repeating units varying in size from disaccharides to heptasaccharides. The final exopolysaccharide of high molecular weight (1 to 2 × 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 little 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 (ie, 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. © 1995 Elsevier/NRA.