bacteroid

Bacteroid is a term used in plant-microbe symbiosis to describe the differentiated, nitrogen-fixing form of rhizobial bacteria that inhabit legume root nodules. In the symbiosis between many leguminous plants and rhizobia, bacteria infect root hairs and multiply within nodules, where they undergo terminal differentiation into bacteroids. This form is adapted for nitrogen fixation rather than free-living growth.

Morphology and physiology of bacteroids often differ from their free-living rhizoid ancestors. They are typically enlarged or irregular in shape and generally lose the ability to replicate, remaining metabolically active to support nitrogen fixation. The transformation involves changes to the cell envelope and metabolism, including the accumulation of storage compounds such as poly-β-hydroxybutyrate in some species. To function effectively, bacteroids operate in a controlled, low-oxygen environment inside nodules, a condition maintained by plant-produced leghemoglobin that buffers oxygen levels to protect the oxygen-sensitive nitrogenase enzyme.

Functionally, the primary role of bacteroids is to fix atmospheric nitrogen (N2) into ammonia (NH3), providing essential nitrogenous compounds to the plant. In return, the plant supplies carbon sources and creates a protected habitat for the bacteroids. This mutualistic interaction contributes significantly to nitrogen input in many terrestrial ecosystems and supports plant growth in nitrogen-poor soils. The extent of bacteroid differentiation and the nature of the host–microbe interaction can vary among legume–rhizobium pairings.