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 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