The form adopted by a nitrogen-fixing bacterium when active within the root nodule of a host plant. For example, Rhizobium bacteria change into enlarged irregularly shaped branching cells when they infect the root cells of their legume host. These bacteroids become surrounded by a peribacteroid membrane, derived from membranes of their host cells, and differentiate to produce the key enzymes and other components of nitrogen fixation, such as nitrogenase. As the infection process progresses, the bacteroids become housed in a root nodule, where they are totally dependent on their host for the energy required for nitrogen. In return they supply their host with an assimilable form of nitrogen, i.e. ammonia, which is incorporated into amino acids. See nitrogen fixation.