The Hyphomicrobiales (synonom Rhizobiales) 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
Accepted families
Aestuariivirgaceae Li et al. 2019
Afifellaceae Hördt et al. 2020
Ahrensiaceae Hördt et al. 2020
Alsobacteraceae Sun et al. 2018
Amorphaceae Hördt et al. 2020
Ancalomicrobiaceae Dahal et al. 2018
Aurantimonadaceae Hördt et al. 2020
Bartonellaceae Gieszczykiewicz 1939 (Approved Lists 1980) Beijerinckiaceae Garrity et al. 2006 Blastochloridaceae Hördt et al. 2020 Boseaceae Hördt et al. 2020
Breoghaniaceae Hördt et al. 2020
Brucellaceae Breed et al. 1957 (Approved Lists 1980) Chelatococcaceae Dedysh et al. 2016 Cohaesibacteraceae Hwang and Cho 2008 Devosiaceae Hördt et al. 2020 Hyphomicrobiaceae Babudieri 1950 (Approved Lists 1980) Kaistiaceae Hördt et al. 2020 Lichenibacteriaceae Pankratov et al. 2020 Lichenihabitantaceae Noh et al. 2019
Methylobacteriaceae Garrity et al. 2006
Methylocystaceae Bowman 2006
Nitrobacteraceae corrig. Buchanan 1917 (Approved Lists 1980)
Notoacmeibacteraceae Huang et al. 2017
Parvibaculaceae Hördt et al. 2020
Phreatobacteraceae Hördt et al. 2020
Phyllobacteriaceae Mergaert and Swings 2006
Pleomorphomonadaceae Hördt et al. 2020
Pseudoxanthobacteraceae Hördt et al. 2020
Rhabdaerophilaceae Ming et al. 2020
Rhizobiaceae Conn 1938 (Approved Lists 1980) Rhodobiaceae Garrity et al. 2006 Roseiarcaceae Kulichevskaya et al. 2014 Salinarimonadaceae Cole et al. 2018 Segnochrobactraceae Akter et al. 2020 Stappiaceae Hördt et al. 2020 Tepidamorphaceae Hördt et al. 2020 Xanthobacteraceae Lee et al. 2005
Unassigned Genera
The following genus has not been assigned to a family: Flaviflagellibacter Dong et al. 2019
Provisional Taxa
These taxa have been published, but have not been validated according to the Bacteriological Code: "Nordella" La Scola et al. 2004 "Propylenellaceae" Liu et al. 2021 "Propylenella" Liu et al. 2021 "Propylenella binzhouense" Liu et al. 2021 "Thermopetrobacter" Sislak 2013
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.