The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families. The four families Nitrobacteraceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae contain at least several genera of nitrogen-fixing, legume-nodulating, microsymbiotic bacteria. Examples are the genera Bradyrhizobium and Rhizobium. Species of the Methylocystaceae are methanotrophs; they use methanol (CH3OH) or methane (CH4) as their sole energy and carbon sources. Other important genera are the human pathogens Bartonella and Brucella, as well as Agrobacterium (useful in genetic engineering).
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature[7] and the phylogeny is based on whole-genome sequences.[2][a]
Natural genetic transformation has been reported in at least four Hyphomicrobiales species: Agrobacterium tumefaciens,[8] Methylobacterium organophilum,[9] Ensifer adhaerens,[10] and Bradyrhizobium japonicum.[11] Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of the donor sequence into the recipient genome by homologous recombination.