【根瘤菌】
大豆的根瘤，每一个都含有数十亿根瘤菌（Bradyrhizobium）。
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。