Hand-colored Scanning-Electron-Micrograph of Rhizobium leguminosarium by Micronaut.
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen which forms an endosymbiotic nitrogen fixing association with roots of legumes. The bacteria colonize plant cells within root nodules where they convert atmospheric nitrogen into ammonia and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant in turn provides the bacteria with organic compounds made by photosynthesis.
Beijerinck in the Netherlands was the first to isolate and cultivate a microorganism from the nodules of legumes in 1888, and named it Bacillus radicicola.