June, 1921. 



SCIENTIFIC AGRICULTURE. 



257 



Nodule Bacteria of Leguminous Plants 



By F. Lohiii.s, Soil Biologist, Bureau of Plant Indus- 

 try. United states Department of Agriculture, 

 and Roy Hansen. Professor of Soils, Univcrsiti/ of 

 Siisl{filrh( irini. Saskatoon. Sask.' 



INTRODUCTION. 



Despite the fact tlhat the nodule bacteria of the 

 lefTuminous plant.s have been made the subject of 

 numerous publications, it is not to be disputed that 

 t.heir true morpholonrical and physiological diaracter, 

 as well as their coi-rect systematic position, are by no 

 means sufficiently known. This is especially clearly 

 demonstrated by the fact that they are still proclaimed 

 liy several writei-s to he the representatives of a 

 special geniLS Rhizobium, once est<iblished by A. B. 

 Frajik as the result of rather inadequate studies upon 

 thi.s subject. In the new classification of bacteria, 

 adopted by the So<Mety of American Bacteriologists, 

 the nodule bacteria again are widely .separated from 

 closely related species, and t.he error concerning the 

 so-called genus Rihizobium has been revived once more. 



Comparative investigations upon the .symbiotic and 

 the non.symbiotic nitrogen-fixing bacteria of the soil, 

 published in 1905 by the .senior autjior, have proved 

 conclusively that tijie nodiile bacteria are not repre- 

 sentatives of a special genus Rhizobium, but that 

 they are closely i-elated to Bacillus radiobacter Bei- 

 jerinek and further to B. lacti.t viscosuni Adametz, 

 n. pnevnioniae Friedliuider, and B. aerogenes Escher- 

 ich. The last tjiree organisms are immotile, while the 

 fiiist one is motile; but here again the very close 

 relation.slhip between the immotile B, aerogenes and 

 the motile B. coli has to he kept in mind. In fact, 

 there can be easily isolated from every .soil numerous 

 varieties of B. radiohacter, which lead gradually up 

 to B. coli, acquiring the power of fermentation and 

 t.hat type of growth on solid substrates which is 

 cliaracteristic of the laf=t-named species. It has l>een 

 pointed nut in detail that all species mentioned above 

 differ only gradually, not principally, in their physio- 

 logical and morphological qualities, and especially that 

 those branched or otherwise changed cell forms which 

 are frequent in tjie root nodules are equally common 

 with all members of this group of eap.sule bacteria, 

 if these are tested adequately." The ability to fix the 

 atmospheric nitrogen was shown to be common in this 

 group of organisms. 



Bacillus radiohacter was found to be peritrichic. 

 and the .siune jiaper also indicate<I (12, p. .592, foot- 

 note)^ that in all jirobability B. radicicola has the 

 same kind of flagellation. But no faultless preparates 

 were obtained at that time. 



• Reprinted from .lournal of Agricultural Research. Vol 

 XX, No. 7. Jan. 3, 1921. 



1 Most of the experiments rtisciis.sed in this paper were 

 made in the summer of 1919 at the University of Tllinois. 

 where at that time the junior author held the i)Osition of 

 A.ssociate in Soil Biology. The protographs accompanying 

 the paper were made by Mr. F. L. Goll. of the Bureau 

 of Plant Industry. United States Department of Agriculture. 



- It is not superfluous to emphasize once m.ore that per- 

 sistence in calling these forms "hacteroids" is by no means 

 to be recommended. They are true bacteria, not foreign 

 bodies looking like bacteria, as Franks' pupil Brunchorst 

 erroneously believed. To speak of a "bacteroid" growth 

 of bacteria is no less absurd than it would be to speak 

 of a "fungoid" growth of fungi. 



In tibe same year, 1905, (I. T. Moore wrote con- 

 cerning the nodule bacteria (14, p. 26) : 



"There does not .seem to be any necessity for 

 creating a new group to include tlhese organisms, a.s 

 has been done by Frank, under the name of Rhizo- 

 bium, for alt^hough there is a certain amount of 

 polymorphism, it is no greater than frequently occurs 

 in the bacteria." 



With regard to the flagellation, however, Moore 

 himself evidently made no special .studies, and, ac- 

 ceptimr Beijerinck's statement that the "swarming 

 bcdies" (gonidia;) of Bacilhis radicicola are mono- 

 trichic as being valid for the bacteria, too. he propased 

 to call the nodule bacteria Pseudomonas radicicola. 

 Numerous aut.hors have followed this .suggestion, and 

 experimentK made by Harrison and Barlow (8) ap- 

 parently confirmed the view that the flag'ellation of 

 these organisms is indeed monotrichic. 



However, these experiments are, in fact, not con- 

 vincing, as has been emphasized especially by Kel- 

 lernian (9~). This author and also G. de Rossi (16. 

 17). Zipfel (191, and Prucha (15) secured results 

 all of whidh demonstrated more or less clearly that 

 \he senior author's as.sumption was correct: Bacillus 

 radicicola is peritrichic; it is no "Pseudomonas." 



But this seemed again to be contradicted by certain 

 resultls obtained by the junior author while workins: 

 with the late T. J. Burrill (61. Numerous tests made 

 with the bacteria isolated from cowpea. .soybean, 

 Japan clover and other plants showed clearly and in- 

 variably monotrieiliic flagellation, and, therefore, the 

 designation Psevdomovas radicicola was: restored once 

 more. Additional results, however, indicated that there 

 are other features which differentiate the bacteria of 

 the cowpea-.soybean group from those li\nng in the 

 roots of clover, alfalfa, pea, and vetc^h. Especially t.ne 

 .slime production and the speed of growi-h appeared 

 to be different, and the organisms studied were ar- 

 ranged into two groups, ".slow growers" and "fast 

 growers". Both, however, were .supposed to be merely 

 varieties of P. radicicola. 



This point remained to be investigated more thor- 

 oughly. In addition, another "fast grower" presented 

 itself for detailed study, which quite regularly ap- 

 peared on thickly sown plates of the ".slow growing" 

 group.s. and w.hidh, indeed, has been mistaken by sev- 

 eral investi£rato7-s as the true nodule organism of 

 cowpea, soybean, Japan clover, etc. Repeatedly such 

 cultures were sent to and tested by the junior author. 

 They were all unable to produce nodules. 



The data ffiven on the following pages make it 

 evident that this "fast grower" is Bacillus radiohacter. 

 which plays in this case, also, a ver>' interesting role. 

 -A,s this same species undoubtedly takes pai-t in many 

 processes occurring in soil and in water, it was 

 t'''on.n-.ht u.seful to srive another more detailed des- 

 '■'■intion of it. especially becau.se, despite its ubiquity, 

 B. radiohacter is much too little known. In addition 



