bacteroids

Bacteroids are the differentiated, nitrogen-fixing forms of rhizobial bacteria that reside inside the nodules of leguminous plants. They originate from rhizobia that have infected host roots and become enclosed within a plant-derived membrane, forming a symbiosome.

In many legume species, bacteroid development is terminal: once differentiated, they cease to divide and remain specialized for nitrogen fixation for the life of the nodule. In other systems, bacteroids can retain limited reproductive potential. Plant-derived peptides, notably nodule-specific cysteine-rich (NCR) peptides, contribute to the differentiation process in many legumes by altering bacterial cell cycle and metabolism.

Within bacteroids, nitrogenase catalyzes the reduction of atmospheric nitrogen (N2) to ammonia, providing a usable source of nitrogen to the plant. This process is highly sensitive to oxygen, so nodules maintain a microaerobic environment with leghemoglobin, a plant oxygen-binding protein that buffers oxygen delivery to the bacteroids.

Energy and carbon for nitrogen fixation are supplied by the plant. Bacteroids receive carbon compounds, such as malate, and ATP through the peribacteroid membrane that surrounds each bacteroid within the plant cell. This energy supports the electron flow required by nitrogenase.

The symbiosis benefits both partners: the plant gains assimilable nitrogen, while rhizobia receive carbon and a habitat. Bacteroids thus form a key component of the legume–Rhizobium mutualism and contribute to natural soil nitrogen inputs.