Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are gram negative, motile, non-sporulating rods.

Diazotrophs are bacteria and archaea that fix atmospheric nitrogen(N2) in the atmosphere into bioavailable forms such as ammonia.