Bacteroids

(1) Large branched cells of nodule-forming bacteria that are contained in nodules on the roots of leguminous plants (clover, alfalfa, and others). The origin of bacteroids is linked to the life cycle of the nodule-forming bacteria, and it occurs in all species. The young cells are rod-shaped. After penetrating into the root hair, they produce nodules and acquire the external appearance characteristic of bacteroids. Bacteroids apparently are more active in fixing atmospheric nitrogen than are young cells.

(2) Strictly anaerobic, gram-negative, highly polymorphic bacteria, which form neither spores nor capsules. They are usually nonmotile rods, as small as 2 microns. They are regularly found in the mouth, intestines, and genitalia of man. Many species are pathogenic and cause acute inflammations.