High Quality Content by WIKIPEDIA articles! The Rhizobiaceae are a family of proteobacteria, including many (but not all) species of rhizobia as well as plant parasites like Agrobacterium. Rhizobiaceae are, like all Proteobacteria, gram-negative. They are aerobic and the cells are usually rod-shaped. Many species of the Rhizobiaceae are diazotrophs, they are able to fix nitrogen and are symbiotic with plant roots.