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What are rhizobia?

October 4, 2011

Rhizobia are legume root-nodule bacteria.  They are soil bacteria that induce the formation of special structures (nodules) on the roots of their host plants.  Inside these nodules, the rhizobia fix nitrogen.  This means that they convert dinitrogen (the nitrogen gas that makes up 80% of the air you breathe) into ammonia.  Ammonia is toxic, so it is rapidly assimilated into organic compounds, most of which the bacteria pass to the plant to fulfil its nutritional need for nitrogen.  Nitrogen fixation is an energy-intensive process, and the bacteria take carbon compounds from the plant to fuel it.  It is a symbiosis, because the bacteria live in intimate association with the plant, and a mutualism, because both partners gain.

That’s it, basically.  Of course, there is much more to say about the diversity of both partners, the way in which they choose each other, the biochemistry of the interaction, the great importance of this nitrogen fixation for ecosystems and agriculture, and so on.  However, it is not my purpose to write a textbook.  There are many thousands of papers, reviews and books on this symbiosis.

If you want to know more, you could start by looking up “rhizobia” on Wikipedia.  The article seems reasonably accurate, though it is very brief and has few references.  Most of these relate to the “sanctions” hypothesis for the evolution of the mutualism, an interesting but tiny corner of the vast literature on rhizobia.  I wonder who wrote the article.  Anyone can write or rewrite a Wikipedia article, so if you want to do the community a service, why not flesh this article out a bit?  Of course, there are other Wikipedia articles that have some relevant information.  In fact, there is also one on “Rhizobium“.

When is a rhizobium not a Rhizobium?

Rhizobia are legume root nodule bacteria.  A rhizobium is a legume root nodule bacterium.  One rhizobium, many rhizobia – just like one bacterium, many bacteria.  You would never make the mistake of writing “a bacteria”, would you?  (I just wish the journalists at New Scientist were as well educated!)  Words like “bacterium” and “rhizobium” are common nouns, like “bread” and “butter”, so they do not have to start with a capital letter.  You can make adjectives from them: “bacterial”, “rhizobial”.  By the way, it’s “rhizobia” not “rhizobiums” because the word is pretending to be Latin.  It is only pretending, because it is actually derived from two Greek words that convey the meaning “root-dweller”.  If it were really derived from Latin words it would not be “rhizobium” but something like “radicola”.

However, “Rhizobium” is a very different thing from “rhizobium”.  It is the formal taxonomic name of a bacterial genus, and you can’t mess with it.  It is always written just like that, in italics with a capital “R“.  You certainly can’t write Rhizobia, because a genus name has no plural form.

Which bacteria are rhizobia?

Back at the beginning of time (or at the beginning of bacterial taxonomy, at any rate), all legume root-nodule bacteria were placed in the genus Rhizobium.  Gradually, it was realised that they were rather diverse and, more importantly, some were more closely related to other, nonrhizobial bacteria.  Some slow-growing rhizobia were split off into a new genus called Bradyrhizobium, and after that the number of bacterial genera that include rhizobia grew rapidly.  Root-nodulating bacteria are increasingly turning up in genera that have already been created to describe other, non-nodulating bacteria, so the genus name is no longer a good guide to whether a bacterium will be a rhizobium.

If you want a recent list of rhizobial species, I recommend the web page maintained by Bevan Weir.  He started it when he was a PhD student (setting up a useful web site is great way to get yourself noticed at the start of your career!).  He’s now a postdoc working on fungi, but still maintains the list of rhizobia.  Thanks, Bevan!  For an authoritative list of all valid bacterial names, not just rhizobia, you should consult the List of Prokaryotic names with Standing in Nomenclature.  There are several long-standing disagreements concerning the taxonomy of rhizobia and their relatives, though, and I shall probably write something about them in future posts.


From → Taxonomy

  1. if a blog is about discussing topics, here is one:

    What do you think about some papers describing rhizobia in the gamma class of Proteobacteria? There has been several papers but none seems to me higly convincing, as it is always one or two isolates among a lot of “common” rhizobium species. It mights exists but a real demonstration would be needed.


  2. A good question, Lionel! I’ll write a post about it.


  3. Megan permalink

    3-way symbiosis?

    I understand that rhizobium and their legume host are mutualistic.
    I also understand that mycorrhizae share a mutualistic relationship with their host.
    I think that both rhizobium and mycorrhizae can be found in the rhizosphere of leguminous plants. If this is true, then what is the relationship between rhizobium and mycorrhizae? Is this a 3-way symbiosis? or not necessarily? It certainly seems that the 3 are all very closely related. Thanks in advance for your help!

  4. Vega permalink

    if accidentally consumed could this bacteria make you sick?

    • Hello Vega

      Rhizobia are bacteria that are adapted to live around plants, and you are not likely to get sick if you accidentally eat a few. Most rhizobia are unable to grow at body temperature, since they normally live in a cooler environment, and they would find conditions in your gut very hostile. Of course, a lot of different bacteria do live in your gut – most of them are harmless and a lot of them are actually important for maintaining your health. In fact, only a very small fraction of all the bacteria in the world would make you sick if you consumed them, but those few that are pathogens of humans can make you very sick indeed, so they attract a lot of attention. I would certainly advise you against consuming bacterial cultures, in case they include some of those pathogens, unless those bacterial cultures are foods such as yogurt or cheese that are deliberately made using bacteria.

  5. Atefa permalink

    What is the difference between Rhizobium and Rhizobacteria? Is it one and the same?

    • Hello Atefa

      Rhizobia and rhizobacteria are not the same thing. A rhizobium is a bacterium that colonises a root nodule on legume plants and fixes nitrogen. A rhizobacterium is any bacterium that is associated with the roots of any plant. Some people restrict the term to bacteria that are beneficial for the plant, but in fact there is a special term for these: plant growth-promoting rhizobacteria, usually called PGPRs. A rhizobium is a particular type of rhizobacterium, but there are also many other kinds of rhizobacteria.

      For an explanation of the distinction between a rhizobium and the genus Rhizobium, see my earlier post:

  6. Renzo Valdez permalink

    It was helpful the page from Weir, congratulations

  7. As a biodynamic farm focused on soil regeneration. We run our sheep through our fruit orchards which are planted with grasses and legumes for, among other reasons, nutrient cycling. We want to get that nitrogen into the soil from the rhizobia dying. Most farms have accomplished this by cutting into and tilling the entire plant into the soil a few times a year. But every time we rip into soil that destroys the fungal pathways and threatens to kill of other bacteria colonies. We instead use a “no till” method by grazing (with animals) or mowing the grass legume cover crops. The hope is that the root of the mowed or grazed legume sloughs off and the rhizobia connected to the now sloughed “dead” root release nitrogen into the soil. Of course, all of this is to reduce the amount of nitrogen we have to input into our soils from outside sources. Is this assumption at all correct?

    • Hello John

      Yes, if you mow or graze off the tops, the roots and nodules will decay and return nutrients to the soil. They do have more nitrogen than average plant roots, but most of the nitrogen fixed in the nodules does not stay below ground. The plant moves it up to make protein in the leaves and eventually in the seeds. That is why legumes are such good forage and beans are so nutritious. If you mow and take away the hay, you are removing some of the nitrogen that has been fixed. If you bring in sheep, a lot of of the nitrogen will go through them and back into the soil, of course.

  8. Ntswaki mogole permalink

    Where rhizobia originate?

  9. olivier permalink

    thank you ,can explain well preparation of Rhizobium an inoculation deeply .

    • If you would like to know how to prepare a rhizobium inoculum, or how to work with rhizobia in general, I recommend a new practical handbook for rhizobium research, published this year:

      Howieson J.G. and Dilworth M.J. (Eds.). 2016. Working with rhizobia. Australian Centre for International Agricultural Research: Canberra.

      You can download it free from

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