Soil Bacteria

How big are bacteria?
What are some of the plant-bacterial associations that fix atmospheric nitrogen?
What are rhizobia?
What is the nodulation process?
What does specificity mean?
What is nitrogenase?
What is leghaemoglobin?
How are rhizobia identified?

Question 1: How big are bacteria?

Most bacteria in soil are about one micron in length or diameter (there are a thousand microns in a millimeter). Some are slightly larger than this, up to several microns, and in rare cases even larger. Their size varies with their environment. Bacteria in environments that have high levels of nutrients may be larger than those in nutrient poor conditions.

The majority of bacteria in soil usually occur as single cells. Bacteria sometimes join together in chains or clusters. They mostly have one of two shapes - spheres (called cocci) and rods (called bacilli). Other bacteria have more varied shapes including spirals and long thin hyphae (although these are less common).

Question 2: What are some of the plant-bacterial associations that fix atmospheric nitrogen?

Associations between bacteria and plants that fix atmospheric nitrogen include an association between species of Frankia bacteria and several tree species such as those of the genera Casuarina and Allocasuarina. Another example is between that of Azospirillum and grasses. The most well-known plant-bacterial associations that fix atmospheric nitrogen is the symbiotic association between rhizobia and legumes.

Question 3: What are rhizobia?

Rhizobia are one of the groups of very small organisms (microorganisms) living in soil. Rhizobia are bacteria which are single cells about one thousandth of a millimetre long.

Rhizobia belong to a specific group of bacteria that form a close association with legume plants. This association is called a symbiosis. The symbiosis results in visible, ball-like structures being formed on roots - these structures are called nodules. The nodules are formed by the plant in response to the presence of the bacteria.

Rhizobia belong to a family of bacteria called Rhizobiaceae. There are a number of groups (genera and species) of bacteria in this family.

The bacteria take nitrogen from the air (which plants cannot use) and convert it into a form of nitrogen called ammonium nitrogen, which plants can use. The process is called nitrogen fixation and these bacteria are often called "nitrogen fixers".

Rhizobia can be extremely beneficial to plants in areas where the soils are low in nitrogen, if they form associations with legumes in these soils.

Rhizobia are found in soils of many natural ecosystems. They may also be present in agricultural areas where they are associated with both crop legumes (like soybean) and pasture legumes (like clover). Usually, the rhizobia in agricultural areas have been introduced when the legumes were sown.

Question 4: What is the nodulation process?

The nodulation process is a series of events in which rhizobia interact with the roots of legume plants to form a specialised structure called a root nodule.

The process involves complicated signals between the bacteria and the roots. In the first stages, the bacteria multiply near the root and then adhere to it. Next, the small hairs on the root's surface curl around the bacteria and they enter the root. Alternatively, the bacteria may enter directly through points on the root surface. The method of entry of the bacteria into the root depends on the type of plant. Once inside the root, the bacteria multiply within thin threads. Signals stimulate cell multiplication of both the plant's cells and the bacteria and this repeated division results in a mass of root cells containing many bacterial cells. Some of these bacteria then change into a form that is able to convert gaseous nitrogen into ammonium nitrogen (that is, they can "fix" nitrogen). These bacteria are then called bacteroids.

The shape the nodules form are controlled by the plant and nodules can vary considerably - both in size and shape.

Most plants need very specific kinds of rhizobia to form nodules. For example, the rhizobia that form nodules on peas cannot form nodules on clover.

Question 5: What does specificity mean?

Specificity is the degree to which a bacteria species can fix atmospheric nitrogen in association with more than one host species. For example, the bacteria Bradyrhizobium japonicum can only form symbiotic associations with soy bean plants and is said to have high specificity. Whereas another bacteria in the genus Bradyrhizobium can form symbiotic associations with both lupins and serradella and so has lower specificity.

Question 6: What is nitrogenase?

Nitrogenase is the bacterial enzyme that allows atmospheric nitrogen to be converted to ammonium. Enzymes are special kinds of proteins that allow reactions to occur at faster speeds than normal or under less extreme conditions. Without nitrogenase, atmospheric nitrogen can only be converted to ammonium with high pressure and temperatures. Nitrogenase requires a low oxygen environment to function.

Question 7: What is leghaemoglobin?

Leghaemaglobin is an organic molecule made up of two protein molecules that control the flow of oxygen to the bacteria that fix atmospheric nitrogen. Leghaemoglobin gives the inside of nodules their redish pink colouring. To see a picture of the inside of a nodule, see Soils are Alive Newsletter Volume 1(3) under 'Newsletter' on soilhealth.com

Question 8: How are rhizobia identified?

Rhizobia are identified according to how fast they grow and multiply on artificial food sources. They are very difficult to identify just from their shape or size alone, because all of them are very small and all are shaped like short rods with rounded ends. So you cannot tell the different types apart just by looking at them, even with a very powerful microscope.

Rhizobia are usually grouped into "fast" and "slow" growers, based on how quickly they grow on artificial food sources.

Recently, new methods of characterisation of rhizobia have been developed using DNA from known rhizobia.

Samples of DNA extracted from soil can be compared with DNA "patterns" that have already been identified for known rhizobia when grown in laboratory conditions. Comparisons are made between sections of the DNA molecule with particular functions (for example, parts of the DNA that are involved with nitrogen fixation) with samples of DNA from soil or from a root nodule. This tells the scientists whether there are genes for that function in the soil sample or within the root nodule.

Serological techniques can also be used. Here, samples of known bacteria are injected into an animal under specific laboratory conditions. The animal's body produces antibodies against the bacteria. (A similar thing happens when we have a vaccination). The component of the blood containing the antibodies is the serum and this is separated from the red blood cells. This is called antiserum.

The antiserum produced by the animal can be collected and used to help locate bacteria in a sample. The antibodies in the antiserum can be joined to a fluorescent dye or to an enzyme with an attached dye. When the antibody is added to the corresponding bacteria it attaches firmly. The bacteria can then be identified using a fluorescence microscope or in a solution that contains a molecule that can interact with the enzyme and cause a colour change in the solution. This method is called the ELISA method.

The antiserum can be chosen to be very specific for particular types of rhizobia, so it is a good way to identify whether those rhizobia are present in a root nodule. It is often important to know whether the rhizobia that were added with the seed have actually been responsible for forming the nodules on the legume or whether other rhizobia already in the soil formed the nodules.

return to the homepage