|Rhizobium tropici on an agar plate.|
R. aggregatum (Hirsch and Müller 1986) Kaur et al. 2011
R. gallicum Amarger et al. 1997
R. giardinii Amarger et al. 1997
R. grahamii López-López et al. 2011
R. leucaenae Ribeiro et al. 2011
R. pseudoryzae Zhang et al. 2011
- See Also Rhizobia
The bacteria colonize plant cells within root nodules; here the bacteria convert atmospheric nitrogen to ammonia and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant provides the bacteria with organic compounds made by photosynthesis.
Beijerinck in the Netherlands was the first to isolate and cultivate a microorganism from the nodules of legumes in 1888. He named it Bacillus radicicola, which is now placed in Bergey's Manual of Determinative Bacteriology under the genus Rhizobium.
The Concept of Cross Inoculation Grouping (C.I.G)
The cross-inoculation grouping based on the classical studies of Fred, Baldwin and McCoy is being generally followed.
The principle of cross-inoculation grouping is based on the ability of an isolate of Rhizobium to form nodules in a limited number of species of legumes related to one another.
All rhizobia that could form nodules on roots of certain legume types have been collectively taken as a species. This system of classification has provided a workable basis for the agricultural practice of legume inoculation. Under this scheme, seven species are generally recognized.
The system of cross-inoculation grouping of rhizobia is not perfect since bacteria have been found to cross-infect or interchange between groups. However, until a better system of classification has been perfected, it appears as if we have to be content with the cross-inoculation grouping as a convenient and workable method of classifying root nodule bacteria into species.
The combined results of both somatic and flagellar reactions have served to distinguish strains within a cross-inoculation group. Serological methods can be used as a means of obtaining information on the distribution of strains that can be recognised within an area, on widely separated areas, on the plant or within a nodule. Serologically, it is known that a single nodule contains a homogeneous population of a single strain of Rhizobium, although it is not uncommon to find more than one strain on the same plant.
Rhizobium forms a symbiotic relationship with certain plants such as legumes. The Rhizobium fixes nitrogen from the air into ammonia, which acts as a natural fertilizer for the plants. Current research is being conducted by Agricultural Research Service microbiologists to discover a way to utilize Rhizobium’s biological nitrogen fixation. This research involves the genetic mapping of various Rhizobium species with its respective symbiotic plant species, like alfalfa or soybean. The goal of this research is to increase the plants’ productivity without using fertilizers. 
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project 
- NOTE: This strain was formerly named Blastobacter aggregatus.
- NOTE: This species was formerly known as R. leguminosarum sv. phaseoli.
- Radeva G, Jurgens G, Niemi M, Nick G, Suominen L, Lindström K. (2001). "Description of two biovars in the Rhizobium galegae species: biovar orientalis and biovar officinalis". Syst. Appl. Microbiol. 24 (2): 192–205. doi:10.1078/0723-2020-00029. PMID 11518322.
- Amarger N, Macheret V, Laguerre G. (1997). "Rhizobium gallicum sp. nov. and Rhizobium giardinii sp. nov., from Phaseolus vulgaris nodules". Int. J. Syst. Bacteriol. 47 (4): 996–1006. doi:10.1099/00207713-47-4-996. PMID 9336898.
- NOTE: R. gallicum and R. mongolense are 99.2% identical in their rDNA and are likely the same species. It has been proposed by Silva et al. that R. mongolense and R. yanglingense be reclassified as R. gallicum sv. orientale.
- Ren DW, Wang ET, Chen WF, Sui XH, Zhang XX, Liu HC, Chen WX (2011). "Rhizobium herbae sp. nov. and Rhizobium giardinii-related bacteria, minor microsymbionts of various wild legumes in China". Int. J. Syst. Evol. Microbiol. 61 (8): 1912–20. doi:10.1099/ijs.0.024943-0. PMID 20833881.
- NOTE: These strains were formerly placed in the genus Agrobacterium.
- Marek-Kozaczuk M, Leszcz A, Wielbo J, Wdowiak-Wróbel S, Skorupska A. (2013). "Rhizobium pisi sv. trifolii K3.22 harboring nod genes of the Rhizobium leguminosarum sv. trifolii cluster". Syst. Appl. Microbiol. doi:10.1016/j.syapm.2013.01.005. PMID 23507586.
- Xu, Lin; Zhang, Yong; Deng, Zheng Shan; Zhao, Liang; Wei, Xiu Li; Wei, Ge Hong (2013). "Rhizobium qilianshanense sp. nov., a novel species isolated from root nodule of Oxytropis ochrocephala Bunge in China". Antonie van Leeuwenhoek 103 (3): 559–565. doi:10.1007/s10482-012-9840-x.
- Turdahon M, Osman G, Hamdun M, Yusuf K, Abdurehim Z, Abaydulla G, Abdukerim M, Fang C, Rahman E (2012). "Rhizobium tarimense sp. nov. isolated from soil in the ancient Khiyik river of Xinjiang, China". Int. J. Syst. Evol. Microbiol. doi:10.1099/ijs.0.042176-0. PMID 23203621.
- Fang Wang, En Tao Wang, Li Juan Wu, Xin Hua Sui, Ying Li Jr., and Wen Xin Chen (2011). "Rhizobium vallis sp. nov., isolated from nodules of three leguminous species". Int. J. Gen. Syst. Evol. Microbiol. 61 (11): 2582–2588. doi:10.2323/jgam.49.155. PMID 12949698.
- J.P. Euzéby. "Rhizobium". List of Prokaryotic names with Standing in Nomenclature (LPSN) . Retrieved 2012-05-02.
- Sawada H, Kuykendall LD, Young JM (2003). "Changing concepts in the systematics of bacterial nitrogen-fixing legume symbionts". J. Gen. Appl. Microbiol. 49 (3): 155–79. doi:10.1099/ijs.0.026484-0. PMID 21131504.
- Sayers et al. "Rhizobium/Agrobacterium group". National Center for Biotechnology Information (NCBI) taxonomy database . Retrieved 2012-05-02.
- All-Species Living Tree Project."16S rRNA-based LTP release 106 (full tree)". Silva Comprehensive Ribosomal RNA Database . Retrieved 2012-05-02.
- This is the type species for the genus.
- Arthrobacter viscosus is currently classified in the Micrococcaceae. See Arthrobacter.