These bacteria were finally isolated and studied in pure culture by Beijerinck. Since then a very great deal of literature has accumulated on the subject of the nodule-producing bacteria, which it is impossible to deal with in a small space. The nodule organism, Bacillus radicicola, when grown on suitable media, passes through a number of different changes in morphology. The most connected account of these changes is given in a paper by Bewley and Hutchinson.[4] In a vigorous culture the commonest type is a rod-shaped bacillus which may or may not be motile. As these get older they often become branched, or irregular in shape, the formation of these branched forms being perhaps due to conditions in the medium. These irregular forms, known as “bacteroids,” are a characteristic type in the nodules. Their production in culture media has been found to be stimulated by sugars and organic acids such as would occur in their environment within the host plant. In the older rods and bacteroids the staining material becomes condensed into granules, and finally the rods disintegrate or break up into coccoid forms. By suitable culture conditions, Bewley and Hutchinson obtained cultures consisting almost entirely of this stage. If such a culture be inoculated into a fresh medium rich in sugar, the swarmer stage appears in great numbers. These swarmers are very minute coccoid rods, ·9 × ·18 in size, that are actively motile. They apparently develop later into the rod stage.
Fig. 6.—Bacillus radicicola. Stages in the life cycle. (After Hutchinson and Bewley.)
Motile Rods
Vacuolated Stage
“Swarmers”
“Bacteroids”
“Pre-swarmers”
Very little is known as to the life of the organism in the soil. It is able, however, to fix nitrogen in cultures, and it has been claimed[35], [48] that it can do so in the soil outside the plant, so that it is possible that we must take it into consideration in this connection. More knowledge is needed as to the optimum conditions for the growth of the organism in the soil. It seems to be more tolerant of acid soil conditions than Azotobacter. The limiting degree of acidity has been found to vary among different varieties of the organism from PH 3·15 to PH 4·9.
A long controversy has been held as to whether the nodule organisms found in different host-plants all belong to one species, or whether there are a number of separate species, each capable of infecting a small group of host-plants. As the term “species” has at present no exact meaning when applied to bacteria, the discussion in this form is unlikely to reach a conclusion. The evidence seems to show that the nodule organisms form a group that is in a state of divergent specialisation to life in different host-plants, and that this specialisation has reached different degrees with different hosts. Thus the organisms from the nodule of the pea (Pisum sativum) will also produce nodules on vicia, Lathyrus, and Lens, but seem to have lost the ability normally to infect other legumes. On the other hand, the bacteria from the nodules of the Soy Bean (Glycine hispida) have become so specialised that they do not infect any other genus of host-plant, and soy beans are resistant to infection by other varieties of the nodule organism. Burrill and Hansen,[6] after an extensive study, divided the nodule bacteria into eleven groups, within each of which the host-plants are interchangeable. The existence of different groups of nodule organisms has been confirmed by the separate evidence of serological tests (Zipfel, Klimmer, and Kruger).[40] The results of cross-inoculation tests have sometimes been conflicting. It seems, indeed, that the host-plant has a variable power of resisting infection, so that when its resistance is lowered it may be capable of infection by a strange variety of the nodule organism. The question that has thus arisen of the ability of the legume to resist infection is of fundamental importance, and its elucidation should throw light on the relation of plants to bacterial infection as a whole.