Difference Between Bradyrhizobium and Rhizobium

The key difference between Bradyrhizobium and Rhizobium is that Bradyrhizobium is a slow-growing N₂ fixing bacterial species while Rhizobium is a fast-growing N₂ fixing bacterial species.

Bradyrhizobium and Rhizobium are gram-negative N₂ fixing soil bacteria. Bradyrhizobium is a rod-shaped bacterium having a single subpolar or polar flagellum. Bradyrhizobium belongs to the family Bradyrhizobiaceae which has 10 genera. On the other hand, Rhizobium is a rod-shaped bacterium having a single polar or 2-6 peritrichous flagella. Rhizobium belongs to the family Rhizobiaceae. Both of these bacteria form root nodules on the host plant.

CONTENTS

1. Overview and Key Difference
2. What is Bradyrhizobium 
3. What is Rhizobium
4. Similarities Between Bradyrhizobium and Rhizobium
5. Side by Side Comparison – Bradyrhizobium vs Rhizobium in Tabular Form
6. Summary

What is Bradyrhizobium?

Bradyrhizobium is a genus of gram-negative soil proteobacteria, and many of these fix atmospheric N₂. N₂ fixation is an important step in the Nitrogen cycle as plants are unable to fix N₂ on their own. Plants cannot use atmospheric N₂ directly. They absorb nitrogen compounds mainly in the form of nitrates. Bradyrhizobium is a rod-shaped and motile bacterium. They are usually soil-dwelling microorganisms that can form symbiotic relationships with leguminous plants such as soya bean and cowpea, where they fix nitrogen in exchange for carbohydrates from plants.

Bradyrhizobium species are a major component of forest soil microbial communities. But strains isolated from these soil microbial communities are incapable of N₂ fixation and nodulation. They are very slow-growing in contrast to Rhizobium species. Bradyrhizobium species normally take 3–5 days to create moderate turbidity and 6–8 hours to double in population size in a liquid medium. When the medium has pentoses as carbon sources, they tend to grow faster. Some strains of Bradyrhizobium species are capable of oxidizing carbon monoxide aerobically. Bradyrhizobium takes atmospheric N₂ and fixes it into NH₃ (ammonia) or NH₄⁺ (ammonium). Bradyrhizobium species have genes such as Nif and fix for N₂ fixation. Furthermore, Bradyrhizobium has over 55 Nod genes that are associated with nodulation. The complete genome size of Bradyrhizobium strain Lb8 is approximately 8.7 Mbp. This genome is a circular chromosome that consists of 8433 protein-coding genes, including one rRNA cluster and 51 t RNA genes.

What is Rhizobium?

Rhizobium is a genus of gram-negative, rod-shaped proteobacteria in soil that fix N₂. They form endosymbiotic N₂ fixation association with roots of legumes and Parasponia. These bacteria colonize plant cells and form root nodules, where they convert atmospheric N₂ into ammonia using nitrogenase enzyme. Then they later provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant, in return, provides organic compounds that are made by photosynthesis to bacteria. Rhizobium acts as a natural fertilizer for the plants.

Some research studies involve genetic mapping of different rhizobial species with their symbiotic plant species, such as alfalfa and soya bean. Rhizobium leguminosarum has a large circular chromosome and five plasmids. The genome size of Rhizobium leguminosarum is approximately 7.7Mbp. They also have important N₂ fixing genes such as nod, fix, and nif, etc., in their genome. In molecular biology, Rhizobium has been identified as a DNA contaminant of extractions kit and in ultrapure water systems.

What are the Similarities Between Bradyrhizobium and Rhizobium?

• Both bacteria belong to the phylum proteobacteria.
• They are gram-negative and rod-shaped bacteria.
• Both are fixing atmospheric N₂.
• The genes such as nod, fix, and nif, etc. for nodulation and N₂
• They are both motile bacteria.
• Both have circular chromosomes.

What is the Difference Between Bradyrhizobium and Rhizobium?

Bradyrhizobium is a slow-growing N₂ fixing bacterial species. In contrast, Rhizobium is a fast-growing N₂ fixing bacterial species. So, this is the key difference between Bradyrhizobium and Rhizobium. However, some forest soil microbial community species of Bradyrhizobium do not fix N₂. But, on the other hand, most of the species of Rhizobium are capable of fixing N₂.

Moreover, Bradyrhizobium has a single subpolar or polar flagellum, while Rhizobium has single polar or 2-6 peritrichous flagella. Thus, this is another significant difference between Bradyrhizobium and Rhizobium.

Below is a summary of the differences between Bradyrhizobium and Rhizobium in tabular form for side by side comparison.

Summary – Bradyrhizobium vs Rhizobium

There is an endosymbiotic relationship between legume and rhizobial species. Rhizobium is fast growing while Bradyrhizobium is slow-growing. Both can be recovered from legume plants like cowpea. Rhizobium and Bradyrhizobium form nodules on the host plant. They carry out N₂ fixation. N₂ fixation is an extremely important process for the host plant. However, some forest soil microbial community species of Bradyrhizobium do not fix N₂. Thus, this summarizes the difference between Bradyrhizobium and Rhizobium.

Reference:

1. “Bradyrhizobium.” Wikipedia, Wikimedia Foundation, 25 Mar. 2021, Available here.
2. “Rhizobium.” Wikipedia, Wikimedia Foundation, 23 Feb. 2021, Available here.

Image Courtesy:

1. “Bradyrhizobium versus Meloidogyne galls” By Scot Nelson (Public Domain) via Flickr
2. “A farmer’s primer on growing soybeans_p61” By IRRI Photos (CC BY-NC-SA 2.0) via Flickr