Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia can't independently fix nitrogen, and requires a plant host. Morphologically they're generally gram negative, motile, non-sporulating rods. History The first species (R. leguminosarum) was identified in 1889, and all further species placed in the Rhizobium genus. However, more advanced methods of analys… (More on Rhizobia)