The extracellular and surface polysaccharides produced by Rhizobium sp
ecies constitute a composite macromolecular interface between the bact
erial cell and its environment. Several of these polysaccharides are i
nvolved in the complex series of interactions leading to the establish
ment of an effective Rhizobium-legume symbiosis. Extracellular heterop
olysaccharides (EPSs) are found in culture supernatants, while capsula
r polysaccharides adhere to the cell surface. Cyclic (1-2)-beta-D gluc
an is a periplasmic oligosaccharide that has also been found in the cu
lture supernatants of some strains. The lipopolysaccharides (LPSs), wh
ich form part of the outer membrane and contain the O-somatic antigens
, comprise the other major group of extracellular polysaccharides. In
this review we will describe the major Rhizobium extracellular structu
res and their role in symbiosis with leguminous plants.