SUMMARY: The monosaccharide composition of extracellular polysaccharides isolated from 101 strains of rhizobia, principally associated with the lotus group of legumes, have been examined. Polysaccharides from 43 fast-growing, acid-producing strains (17 lotus) gave a substantially consistent monosaccharide pattern of glucose, uronic acid and galactose with little or no mannose. Polysaccharides from lucerne and goat's rue strains (four) of this class of rhizobia differed in that they contained almost no uronic acid. Fifty-eight slow-growing, non-acid-producing strains (52 lotus) were also investigated. On centrifuging, 36 of them gave a mucilagenous clot of cells and polysaccharide and required dilute alkali treatment to separate the polysaccharide. No polysaccharide was produced by four non-acid-producing strains. Polysaccharides from the non-acid-producing rhizobia gave much more complex monosaccharide patterns. Glucose and mannose were nearly always present in large amounts; galactose and uronic acid were frequently absent while fucose, rhamnose and three unidentified sugars were occasionally present. Electron micrograph studies showed extracellular material which, in strains giving a clot, was not removed by centrifuging. Monosaccharide composition of the polysaccharides appeared to be correlated with the division into acid-and non-acid-producing classes of rhizobia. Both classes of rhizobial strains associated with Lotus species conformed to this correlation apart from one strain which was intermediate.


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