Rhizobia

Rhizobia are soil bacteria which become established inside root nodules of legumes (the pea family: Fabaceae). 
They fix nitrogen in a form the plant can use. Rhizobia require a plant host; they cannot independently fix nitrogen. The relationship is a classic example of symbiosis and mutualism.
The word rhizobia comes from the Ancient Greek "ῥίζα" (rhiza) for root and "βίος" (bios) for life.