SUCCINOGLYCAN AND GALACTOGLUCAN

Succinoglycan and galactoglucan, also known as EPS-I and EPS-II, respe ctively, have much been studied in Rhizobium meliloti isolated from th e root nodules of lucerne. These polysaccharides have also been obtain ed from a number of other bacteria, which have been isolated from dive rse sources of soil and water communities. In 1965 Harada et al. isola ted from soil a bacterium named Alcaligenes faecalis var. myxogenes wh ich produces a polysaccharide that for the first time was characterize d and named as 'succinoglycan'. Succinoglycans were found to be synthe sized normally by strains of Agrobacterium tumefaciens, A. radiobacter and Rhizobium meliloti and were also produced by many strains of Pseu domonas spp. isolated from activated sludge and from other sources. Th ey had component compositions of D-glucose, D-galactose, pyruvate, suc cinate and acetyl in the ratios 7:1:1:1:1. Because of their useful app lications as an emulsion-stabilizing, suspending and thickening agent, succinoglycans were produced industrially by Shell. Galactoglucan-pro ducing bacteria were first encountered in activated sludge in 1974 and could reproducibly be isolated from water sediments and from biofilms in water streams and were identified as Agrobacterium radiobacter and Pseudomonas spp. On isolation of these bacteria, colonies were recogn ized by their very viscous appearance on agar plates. Polymers were fo und to have very simple component compositions of D-glucose, D-galacto se and pyruvate in the ratios 1:1:1. These highly pyruvylated polymers , which were named 'galactoglucans', were structurally identified as h aving a disaccharide repeating unit of alternatively (1-->3)-linked D- glucose and D-galactose residues. Surprisingly, Rhizobium meliloti mut ant strains were found, which overproduced galactoglucans instead of n ormal succinoglycan. The osmotic conditions of the growth media, in wh ich the bacteria were cultivated, were found to have a profound effect on the direction of synthesis of the succinoglycan/galactoglucan olig omeric and polymeric products and it was proposed that these products are related by a common biosynthetic pathway (presented in part at the Beijerinck Centennial Congress in The Hague, The Netherlands, 10-14 D ecember 1995). (C) 1997 Elsevier Science Ltd.