Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


1. In the experimental work reported in this bulletin studies were made of some physiological activities of the nitrogen fixing bacteria of the genus Rhizobium, to ascertain whether or not certain tests could be used for diagnostic purposes in separating the various cross-inoculation groups.

2. The studies made include agglutination, gelatin liquefaction and fermentation of sugars and other carbon compounds.

3. The organisms used were chiefly of the alfalfa and sweet clover, soybean, pea, red clover and dalea groups. Bacillus radiobacter was also studied for purposes of comparison.

4. In this work agglutination occurred in all homologous tests.

5. In the heterologous tests agglutination occurred only when the organisms in question had been isolated from plants belonging to the same cross-inoculation groups.

6. The results secured furnish strong evidence of a direct correlation between the groups of legume bacteria when they are classed according to their ability to cross-inoculate, and when they are grouped according to their serological reactions in cross-agglutination.

7. Altho most of the strains tested did not possess the ability to liquefy gelatin, a few strains did.

8. The data secured do not show any correlation between the power to liquefy gelatin and the ability to inoculate.

9. The results of the fermentation studies in synthetic nitrogen- free media indicate that the legume bacteria attack monosaccharide sugars with a greater production of acid than they do the disaccharides or trisaccharides, the polyatomic alcohol dulcitol, the benzene derivative inositol or the polysaccharide dextrin.

10. The results of these studies show, further, that there is a wide variation among organisms of the same cross-inoculation group when they are compared upon the basis of their ability to change the reaction of sugar media.

11. The variations among strains within a single cross-inoculation group in this respect are as great or greater than the differences between organisms of different groups.

12. The growth of most of the organisms tested in media containing nitrogen as peptone caused the reaction to become more alkaline.

13. The variations in the amount of acid or alkali produced in peptone media were wide, and as great between strains of a single group as between strains of different groups.

14. In general, the soybean bacteria showed rather weak fermentative powers, and this was also true of the strain of red clover bacteria tested. Some strains of the alfalfa bacteria and the pea bacteria showed rather strong fermentative powers.

15. There are some indications that the fermentation tests might be of value in characterizing and differentiating cross-inoculation groups of legume bacteria. 16. The physiological characters of Bacillus radiobacter, so far as they were tested, are not sufficiently different from those of the legume bacteria to permit their separation by these tests.



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