Hyphomicrobiales

The Hyphomicrobiales are an order of Gram-negative Alphaproteobacteria.
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).
Taxonomy.
Provisional Taxa.
These taxa have been published, but have not been validated according to the Bacteriological Code.
Phylogeny.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and the phylogeny is based on whole-genome sequences.
Natural genetic transformation.
Natural genetic transformation has been reported in at least four "Hyphomicrobiales" species: "Agrobacterium tumefaciens", "Methylobacterium organophilum", "Ensifer adhaerens", and "Bradyrhizobium japonicum". 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.