根瘤菌主要指与豆类作物根部共生形成根瘤并能固氮的细菌，一般指根瘤菌属和慢生根瘤菌属；两属都属于根瘤菌目。

根瘤菌侵入寄主根内，刺激根部皮层和中柱鞘的某些细胞，引起这些细胞的强烈生长，使根的局部膨大形成根瘤；根瘤菌在根内定居，植物供给根瘤菌以矿物养料和能源，根瘤菌固定大气中游离氮气，为植物提供氮素养料，两者在拮抗寄生关系中处于均衡状态而表现共生现象。

根瘤菌细胞呈杆状，有鞭毛和荚膜，鞭毛为正常的活动器官，无芽孢，革兰氏染色阴性，属需氧型。在根瘤中生活的菌体形式多种多样，有梨形、棍棒型或T、X、Y等型，这种变形的菌体成为类菌体；类菌体在豆科植物体内不生长繁殖，却能与豆科植物共同固氮，对豆科植物有良好作用。

每种根瘤菌只能与一种或几种植物建立共生关系，如下表：

此外，还有些微生物能同非豆类植物共同形成根瘤固氮，主要是一些放线菌（如：小头木麻黄、赤杨、杨梅）、蓝绿菌（如：苏铁、竹柏、满江红）。

下属物种
 根瘤菌属 Rhizobium Frank, 1889
 苜蓿根瘤菌
 百脉根瘤菌
 Rhizobium acidisoli Román-Ponce et al. 2016
 Rhizobium aegyptiacum Shamseldin et al. 2017
 Rhizobium aethiopicum Aserse et al. 2017
 Rhizobium aggregatum (Hirsch and Müller 1986) Kaur et al. 2011
 Rhizobium alamii Berge et al. 2009
 Rhizobium alkalisoli Lu et al. 2009，异名
 Rhizobium altiplani Baraúna et al. 2016
 Rhizobium alvei Sheu et al. 2015
 Rhizobium anhuiense Zhang et al. 2015
 Rhizobium aquaticum Máthé et al. 2019
 Rhizobium arenae Zhang et al. 2017
 Rhizobium arsenicireducens Mohapatra et al. 2020
 Rhizobium azibense Mnasri et al. 2014
 Rhizobium azooxidifex Behrendt et al. 2016
 Rhizobium bangladeshense Rashid et al. 2015
 Rhizobium binae Rashid et al. 2015
 Rhizobium borbori Zhang et al. 2011，异名
 Rhizobium calliandrae Rincón-Rosales et al. 2013
 Rhizobium capsici Lin et al. 2015
 Rhizobium cauense Liu et al. 2015
 Rhizobium cellulosilyticum García-Fraile et al. 2007
 Rhizobium chutanense Huo et al. 2019
 Rhizobium ciceri Nour et al. 1994，异名
 Rhizobium daejeonense Quan et al. 2005
 Rhizobium dioscoreae Ouyabe et al. 2020
 Rhizobium ecuadorense Ribeiro et al. 2015
 Rhizobium endolithicum Parag et al. 2014
 Rhizobium endophyticum López-López et al. 2011
 Rhizobium esperanzae Cordeiro et al. 2017
 Rhizobium etli Segovia et al. 1993
 Rhizobium fabae Tian et al. 2008
 Rhizobium favelukesii Torres Tejerizo et al. 2016
 Rhizobium flavum Gu et al. 2014
 Rhizobium fredii Scholla and Elkan 1984，异名
 Rhizobium freirei Dall'Agnol et al. 2013
 Rhizobium galegae Lindström 1989，异名
 Rhizobium gallicum Amarger et al. 1997
 格氏根瘤菌 Rhizobium gei Shi et al. 2016
 Rhizobium giardinii Amarger et al. 1997
 Rhizobium grahamii López-López et al. 2012
 海南根瘤菌 Rhizobium hainanense Chen et al. 1997
 Rhizobium halophytocola Bibi et al. 2012
 Rhizobium helianthi Wei et al. 2015
 Rhizobium herbae Ren et al. 2011，异名
 Rhizobium hidalgonense Yan et al. 2020
 华氏根瘤菌 Rhizobium huakuii Chen et al. 1991，异名
 Rhizobium huautlense Wang et al. 1998，异名
 Rhizobium indigoferae Wei et al. 2002
 Rhizobium ipomoeae Sheu et al. 2016
 Rhizobium jaguaris Rincón-Rosales et al. 2013
 Rhizobium japonicum (Kirchner 1896) Buchanan 1926，异名
 Rhizobium laguerreae Saïdi et al. 2014
 Rhizobium larrymoorei'' (Bouzar and Jones 2001) Young 2004，异名
 豌豆根瘤菌 Rhizobium leguminosarum (Frank 1879) Frank 1889
 Rhizobium lemnae Kittiwongwattana and Thawai 2014
 Rhizobium lentis Rashid et al. 2015
 Rhizobium leucaenae Ribeiro et al. 2012
 Rhizobium loessense Wei et al. 2003，异名
 Rhizobium loti Jarvis et al. 1982，异名
 Rhizobium lupini (Schroeter 1886) Eckhardt et al. 1931，异名
 Rhizobium lusitanum Valverde et al. 2006
 Rhizobium marinum Liu et al. 2015
 Rhizobium mayense Rincón-Rosales et al. 2013
 Rhizobium mediterraneum Nour et al. 1995，异名
 Rhizobium meliloti Dangeard 1926，异名
 Rhizobium mesoamericanum López-López et al. 2012
 Rhizobium mesosinicum Lin et al. 2009
 Rhizobium metallidurans Grison et al. 2015
 Rhizobium miluonense Gu et al. 2008
 Rhizobium mongolense Van Berkum et al. 1998
 Rhizobium multihospitium Han et al. 2008
 Rhizobium naphthalenivorans Kaiya et al. 2018
 Rhizobium nepotum Puławska et al. 2012，异名
 Rhizobium oryzae Peng et al. 2008，异名
 Rhizobium oryzicola Zhang et al. 2015
 Rhizobium oryziradicis Zhao et al. 2017，异名
 Rhizobium pakistanense corrig. Khalid et al. 2015
 Rhizobium pakistanensis Khalid et al. 2015，拼写错误
 Rhizobium paknamense Kittiwongwattana and Thawai 2013
 Rhizobium paranaense Dall'Agnol et al. 2014
 Rhizobium petrolearium Zhang et al. 2012
 Rhizobium phaseoli Dangeard 1926 Rhizobium pisi Ramírez-Bahena et al. 2008
 Rhizobium populi Rozahon et al. 2014
 Rhizobium pseudoryzae Zhang et al. 2011，异名
 Rhizobium puerariae Boonsnongcheep et al. 2016
 Rhizobium pusense Panday et al. 2011，异名
 Rhizobium radiobacter (Beijerinck and van Delden 1902) Young et al. 2001，异名
 Rhizobium rhizogenes (Riker et al. 1930) Young et al. 2001
 Rhizobium rhizophilum Gao et al. 2020
 Rhizobium rhizoryzae Zhang et al. 2014
 Rhizobium rosettiformans Kaur et al. 2011
 Rhizobium rubi (Hildebrand 1940) Young et al. 2001，异名
 Rhizobium ruizarguesonis Jorrin et al. 2020
 Rhizobium selenireducens Hunter et al. 2008
 Rhizobium selenitireducens corrig. Hunter et al. 2008
 Rhizobium skierniewicense Puławska et al. 2012
 Rhizobium smilacinae Zhang et al. 2015
 Rhizobium soli Yoon et al. 2010
 Rhizobium sophorae Jiao et al. 2015
 Rhizobium sophoriradicis Jiao et al. 2015
 Rhizobium sphaerophysae Xu et al. 2012
 Rhizobium straminoryzae Lin et al. 2014
 Rhizobium subbaraonis Ramana et al. 2013
 Rhizobium sullae Squartini et al. 2002
 太白山根瘤菌 Rhizobium taibaishanense Yao et al. 2012，异名
 Rhizobium tarimense Turdahon et al. 2013
 天山根瘤菌 Rhizobium tianshanense Chen et al. 1995，异名
 Rhizobium tibeticum Hou et al. 2009
 Rhizobium trifolii Dangeard 1926，异名
 Rhizobium tropici Martínez-Romero et al. 1991
 Rhizobium tubonense Zhang et al. 2011
 Rhizobium tumorigenes Kuzmanović et al. 2019
 Rhizobium undicola (de Lajudie et al. 1998) Young et al. 2001，异名
 Rhizobium vallis Wang et al. 2011
 Rhizobium vignae Ren et al. 2011，异名
 Rhizobium viscosum (Gasdorf et al. 1965) Flores-Félix et al. 2017
 Rhizobium vitis (Ophel and Kerr 1990) Young et al. 2001，异名
 Rhizobium wenxiniae Gao et al. 2017
 Rhizobium wuzhouense Yuan et al. 2018
 Rhizobium yanglingense Tan et al. 2001
 Rhizobium yantingense Chen et al. 2015
 Rhizobium zeae Celador-Lera et al. 2017
 慢生根瘤菌属 Bradyrhizobium Jordan，1982
 大豆慢生根瘤菌 Bradyrhizobium japonicum''，曾称为大豆根瘤菌。

参考资料

外部链接 
 Current research on Rhizobium leguminosarum  at the Norwich Research Park
 Video and commentary on root nodules and Rhizobium in White Clover 

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