Members of the genus have been classified in the order on the basis of morphology, cell chemistry, and 16S rRNA sequences and catalogs. This genus, which is presently defined by morphology, cell chemistry, the ability to fix nitrogen, and infectivity for and ability to enter into symbiotic relationships with certain plant hosts, may be heterogeneous. species groups have been difficult to delineate by classical phenotypic methods. The recent use of DNA-DNA pairing and low-frequency restriction fragment analysis, as well as probes composed of certain sequences from the (nitrogen fixation) genes or the variable regions of 16S rRNA, has contributed to substantial progress in developing species concepts. In this review I trace the taxonomic history of the genus and outline some of the problems to be resolved in the future.


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