FAMILY- RmzaMACH^E-- 223 



FAMILY IV. RHIZOBL\CEAE COXX. 



(Jour. Bact., 36, 1938, 321.) 

 Cells without endospores, rod-shaped, sparsely flagellated (one polar or lateral flagel- 

 lum, or 2 to 4 peritrichous ones) ; some species non-motile. Usually Gram-negative. 

 One genus (Chrotnobacterium) produces a violet pigment. Grow aerobically on ordi- 

 nary culture media containing glucose. Glucose and sometimes other carbohydrates 

 are utilized, without appreciable acid formation. Saprophytes, symbionts and 

 pathogens. The latter are usually plant pathogens forming abnormal growths on 

 roots and stems. 



Key to genera of family Rhizobiaceae. 



I. Cells capable of fixing free nitrogen when growing symbiotically on the roots of 

 Leguminosae. 



Genus I. Rhizobium, p. 223. 

 II. Either plant pathogens which attack roots or produce hypertrophies on stems ; or 

 free-living non-chromogenic soil or water forms. Do not fix nitrogen. 



Genus II. Agrobacterium, p. 227. 

 III. Usually free-living soil and water forms which produce a violet chromogenesis. 



Genus III. Chroniobacterium, p. 231. 



Genus I. Rhizobium Frank.* 



{Phytomyxa Schroeter, in Cohn, Kryptogamen-Flora von Schlesien, S, 1886, 134; 

 Frank, Ber. d. deut. bot. Gesellsch., 7, 1889, 380; Rhizobacterium Kirchner, Beitr. z. 

 Biol. d. Pflanzen, 7, 1895, 221; Rhizomonas Orla-Jensen, Cent. f. Bakt., II Abt., 22, 

 1909, 328.) From Greek rhiza, root; bios, life. 



Rods: 0.5-0.9 by 1.2-3.0 microns. Motile when young, commonly changing to bac- 

 teroidal forms (a) upon artificial culture media containing alkaloids or glucosides, or 

 in which acidity is increased; or (b) during symbiosis within the nodule. Gram- 

 negative. Aerobic, heterotrophic, growing best with extracts of yeast, malt or other 

 plant materials. Xitrates may be reduced to nitrites. Nitrites are not utilized. 

 Gelatin is not liquefied or is very slightly liquefied after long incubation. Optimum 

 temperature 25°C. This group is capable of producing nodules on the roots of 

 Leguminosae, and of fixing free nitrogen during this symbiosis. 



The type species is Rhizobium leguminosarum Frank. 



Key to the species of genus Rhizobium. 

 1. Litmus milk alkaline. 



a. Formation of serum zone in milk. 



b. Moderate growth, slight acid reaction on yeast water agar plus mono-, di- 

 and trisaccharides. 



c. Causes formation of root nodules on species of the genera Lathyrus, 

 Pisum, Vicia and Lens. Bacteroids irregular with x, y, star-, and 

 club-shaped forms; rods peritrichous when young. 



1. Rhizobium leguminosarum. 



cc. Causes formation of root nodules on Phaseolus vulgaris, P. multiflorus 

 and P. angustif alius . Bacteroids vacuolated rods, few branched 

 forms ; j'oung cells peritrichous. 



2. Rhizobium phaseoli. 



* The genus Rhizobium was revised by Dr. and Mrs. O. N. Allen under the direction 

 of Prof. E. B. Fred and Prof. I. L. Baldwin, Univ. of Wisconsin, Madison, Wis., Jan., 

 1938; further revision by Dr. O. X. Allen, Jan., 1943. 



