rhi·zo·bi·um \rīˈzōbēəm\noun Etymology: New Latin, from rhiz- + -bium 1.capitalized: the genus of Rhizobiaceae comprising small heterotrophic soil bacteria capable of forming symbiotic nodules on the roots of leguminous plants and of there becoming bacteroids that fix atmospheric nitrogen 2.pluralrhizo·bia\-ēə\often capitalized: any bacterium of the genus Rhizobium