Succinoglycan and galactoglucan

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Succinoglycan and galactoglucan, also known as EPS-I and EPS-II, respectively, have much been studied in Rhizobium meliloti isolated from the root nodules of lucerne. These polysaccharides have also been obtained from a number of other bacteria, which have been isolated from diverse sources of soil and water communities. In 1965 Harada et al. isolated from soil a bacterium named Alcaligenes faecalis var. myxogenes which produces a polysaccharide that for the first time was characterized and named as ‘succinoglycan’. Succinoglycans were found to be synthesized normally by strains of Agrobacterium tumefaciens, A. radiobacter and Rhizobium meliloti and were also produced by many strains of Pseudomonas spp. isolated from activated sludge and from other sources. They had component compositions of d-glucose, d-galactose, pyruvate, succinate and acetyl in the ratios 7:1:1:1:1. Because of their useful applications as an emulsion-stabilizing, suspending and thickening agent, succinoglycans were produced industrially by Shell. Galactoglucan-producing 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 recognized by their very viscous appearance on agar plates. Polymers were found to have very simple component compositions of d-glucose, d-galactose and pyruvate in the ratios 1:1:1. These highly pyruvylated polymers, which were named ‘galactoglucans’, were structurally identified as having a disaccharide repeating unit of alternatively (1→3)-linked d-glucose and d-galactose residues. Surprisingly, Rhizobium meliloti mutant strains were found, which overproduced galactoglucans instead of normal succinoglycan. The osmotic conditions of the growth media, in which the bacteria were cultivated, were found to have a profound effect on the direction of synthesis of the succinoglycan/galactoglucan oligomeric 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 December 1995).