Rhizobia are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). Rhizobia require a plant host; they can't independently fix nitrogen. Morphologically, they're generally gram negative, motile, non-sporulating rods. History The first species of rhizobia, R. leguminosarum, was identified in 1889, and all further species were initially placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classif… (More on Rhizobia)