HYPOOSMOTIC ADAPTATION IN RHIZOBIUM-MELILOTI REQUIRES BETA-(1-]2)-GLUCAN

β-(1→2)-Glucan, an unusual cyclic oligosaccharide, can be isolated from the periplasm of bacteria belonging to the family Rhizobiaceae. Data presented here suggest that the periplasmic β-(1→2)-glucan of Rhizobium meliloti plays a major role in osmotic adaptation. First, growth of R. meliloti in a low-osmolarity medium causes a large accumulation of periplasmic β-(1→2)-glucan. Second, mutations in the ndv genes, which prevent this accumulation of β-(1→2)-glucan, reduce cell growth rates under low-osmolarity conditions and cause several other phenotypic changes indicative of an altered or stressed surface. Third, growth of the ndv mutants can be restored by raising the osmolarity of the medium with the addition of a variety of ionic or nonionic compounds. The phenotypic changes associated with the cell surface of the mutants can also be substantially suppressed by increasing the medium osmolarity. On the basis of these data and general considerations about the periplasmic space in gram-negative bacteria, we suggest a mechanism of hypoosmotic adaptation in R. meliloti in which β-(1→2)-glucan plays an essential role.