Rhizobium  

What you should know:

Rhizobium is a Gram-negative rod-shaped bacterium that lives as a symbiont in the root nodules of leguminous plants (alfalfa, pea). There, it carries out nitrogen fixation for the plant in return for minerals and organic matter supplied by the plant.

Classification: 

 Domain Bacteria

    Phylum Proteobacteria 

        Class α - Proteobacteria 

Bergey's Manual Volume 2: The Proteobacteria