Freeze-etch study of the cell envelope of cowpea rhizobia: compartmentalization of the periplasmic space in relation to polysaccharide excretion

Abstract

Freeze-etching electron microscopy of free-living cowpea rhizobia revealed distinctive features of the cell envelope of this bacteria. The topology of the outer and cytoplasmic membranes was comparable with that described for other Gram-negative bacteria. In cowpea Rhizobium strains JLn(c) and NC-92, the rigid cell wall almost invariably cleaved away from the outer membrane, thus exposing two distinct fracture faces in the outer membrane, which to date have been ill defined in rhizobia. Cross-fractured cells showed enlargement of the periplasmic region as a result of invagination of the cytoplasmic membrane near one end of the cell. This feature was consistent in all exponential and stationary phase cells. Ruthenium red staining of cells in various growth phases showed unipolar initiation of the capsule. A unique structure seen within the enlarged periplasm was the presence of a smooth lipidic vesicle. Key words: Rhizobium, freeze-etch, membrane, polysaccharide, vesicle.