Species vs. strains - what’s the difference?

Because nitrogen inoculants work by infecting a plant root, each pulse crop associates with a different Rhizobia species. For example, pea and lentil will only form nodules when inoculated with Rhizobium leguminosarum; alfalfa and sweetclover when inoculated with Sinorhizobium meliloti.

Once you have selected the right Rhizobium species, you must choose the inoculant that contains the best strain of that species since strains can vary greatly in the amount of nitrogen they fix.


Building a Better Rhizobial Inoculant

By: Dr. J. Kevin Vessey Dean, Faculty of Graduate Studies and Research Professor of Biology Saint Mary’s University (Formerly Professor, University of Manitoba)

Did you ever wonder how inoculant companies find the rhizobia for legume inoculants? It is a complex and very important process. To maximize nitrogen accumulation in legume crops, rhizobia selection is one of the most important determining factors.

The symbiotic relationship between legumes and the rhizobia bacteria is a very complex partnership. Only certain species of rhizobia will infect and fix nitrogen in certain species of legumes. Soybean rhizobia will only infect soybeans; not pea, common bean or lentils. This is an easy distinction; the much tougher choice is to select one or two strains from the millions of different strains of rhizobia that could be found in the world.

In bacterial lingo, the term “strain” refers to bacteria within the same species, but with different genetic make-ups (in the same way varieties are different genotypes of the same species of crop). “Strain selection”, is the process of finding “the best” strain of rhizobia to use.

Important factors in selecting strain
What factors do inoculant companies look for when they do strain selection? Firstly, one needs a strain of rhizobium which is very “infective”, meaning when the rhizobia are placed on or near the legume seed at planting, they will be very good at out-competing other bacteria in the soil, infecting the plant, and forming nodules. A second important factor is the strain must be very “effective”, meaning the strain must be very good at fixing nitrogen. Not all rhizobia are created equal when it comes to fixing nitrogen. Therefore, inoculant companies look for strains which are both infective and effective.

Some complicating factors in finding rhizobia that are both infective and effective on certain legumes are the differences in the genetic make-up of the host legume. A strain of rhizobia might be great in one variety of soybean, but bad in another. We refer to this as “host-strain interactions”. Other important factors on strain selection are the effects of soil and climate. A strain of alfalfa rhizobia that might work well in the acidic soils and hot climate of Australia may not do well in the alkaline soils and cooler climate of western Canada. Therefore, inoculant companies have to make sure their rhizobial strains work well in many varieties of the legume, and in the types of soils and climates where the inoculant will be sold.

Host-strain interactions and the influence of soil and climate are important factors in strain selection, but not the only ones. Other important factors in selecting strains for inoculants include:
• Capability to be produced in large quantities under industrial conditions
• Survivability in the inoculant (“shelf life”)
• Resistance to bacterial and fungal seed treatments
• Genetic stability (i.e. low rate of genetic conversion that might result in loss of infectiveness or effectiveness)

The process of selecting inoculant strains
After reading the above factors, one could imagine finding “the best” strain to go into a commercial inoculant is akin to looking for a needle in a haystack. There are no shortage of needles in this haystack; it’s just that very few of the needles are sharp. That is, one can find MANY rhizobial strains to test, but ones that meet all the requirements for a good inoculant strain are few and far between.

Finding good inoculant strains involves a two step process. One first has to collect rhizobial strains to test, and then do much laboratory, greenhouse and field testing to select the best strains from the collection.

There are two general approaches to collecting rhizobial strains to test for their inclusion in inoculants. I call these the “shotgun” and the “nature-assisted” approaches.

The shotgun approach is one of finding strains from a wide variety of sources. There are rhizobial collections in many laboratories around the world. Strains from these collections may be purchased or donated. Likewise, many rhizobial strains can be found in the soil of almost any field that has grown the crop before. One can isolate these rhizobia by sampling the bacteria population from the soil, then growing the bacteria on “selective media” allowing only the rhizobial species of interest to grow. In the shotgun approach, the researcher is making few or no pre-judgements on which rhizobial strains might be best for their particular inoculant. This approach draws from a larger genetic background of rhizobia, but it can be very time consuming to identify the best strains.

The nature-assisted approach uses host legume plants to help collect rhizobia for testing. This process involves planting desired varieties of uninoculated legumes in fields which have previously grown the crop and collecting rhizobia from plant nodules. By selecting from large healthy nodules, the researcher has used nature to help identify rhizobia which are infective in the desired legume varieties. One might think that strains of rhizobia previously used to inoculate the crops would be “re-isolated” by this process. However, rhizobia are rarely genetically stable when living in the soil with many mutations occurring in the rhizobia even in a single season. Some of these mutations may lead to more infective and effective strains of rhizobium than existed in the original rhizobia used to inoculate the crop.

Once a rhizobial collection of dozens, or even hundreds, of strains have been put together, the real work begins. Through a series of tests in culture, in plants grown in greenhouses and finally in test plots in the field, researchers narrow down the collections into one or more strains which are superior in infectivity and effectivity to currently available inoculant strains.

Considerable work and resources go into building a better rhizobial inoculant. As varieties and growing conditions for legume crops change, the search for “new and improved” strains of rhizobia for legume inoculants continues to be an ongoing job for inoculant companies.

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• Species vs strain
• Survival and Effectiveness