Rhizobacteria

thumb|right|300px|Cross section though a soybean (Glycine max 'Essex') root nodule: The rhizobacteria, Bradyrhizobium|Bradyrhizobium japonicum, colonizes the roots and establishes a nitrogen-fixing symbiosis. This high-magnification image shows part of a cell with single bacteroids within their host plant. In this image, endoplasmic reticulum, dictysome, and cell wall can be seen.

Rhizobacteria are root-associated bacteria that can have a detrimental (parasitic varieties), neutral or beneficial effect on plant growth. The name comes from the Greek rhiza, meaning root. The term usually refers to bacteria that form symbiotic relationships with many plants (mutualism). Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature.