The life-cycle of the nodule organism, bacillus radicicola (Beij.), in soil and its relation to the infection of the host plant

The existence of changes in the form of Bacillus radicicola has been known since Beijerinck (2) first isolated it in 1888 from leguminous plant nodules. He observed the motile “swarmer” stage as well as the branching forms, whose nature was already the subject of controversy. About the same time the development of straight-rod forms of the organism was described by Prazmowski (14). Numerous writers have since observed the existence of the organism in the three conditions of straight rods, branching rods and cocci (for references, see Löhnis, 1921 (10)). In 1916 Löhnis and Smith (11) claimed that the various forms constituted a definite life-cycle through which the organism normally passes, and this cycle, as seen in cultures, was carefully described in 1919 by Bewley and Hutchinson (3). In a vigorous young culture, the predominating form of the organism is a short, evenly staining rod (fig. 1). These rods soon undergo a change in internal structure, the staining material becoming segregated into bands crossing the cell. During this banded stage the cells frequently become swollen, distorted, and branched, the so-called “bacteroids” (Brunchorst (4)), but this irregularity of form is not an essential part of the life-cycle, but would appear to be a response to conditions of the environment (Buchanan, 1909) (5). The banded cells give rise to the cocci by further condensation of the bands. The origin of the cocci within the mother-cell was described and illustrated in 1891 by Morck (12), who was the first to appreciate the relation of the internal structure of the cell to the life-history of the organism. The cocci are usually released in a non-motile condition, and afterwards develop flagella, becoming actively motile, the “swarmers” of Beijerinck (2). Under certain conditions, however, the cocci develop flagella while still enclosed within the mother-cell. This condition has been described by Greig-Smith (8) and the observation confirmed by one of the present authors (7). The cocci eventually become elongated and thus pass into the unbanded rod stage. The flagella, which are developed on the cocci, persist after this elongation, but are soon lost: the rods then become non-motile. The development of motility in a culture is thus intimately associated with the appearance of the coccus stage.
RESP-603