Ensifer medicae (also known as ''Sinorhizobium medicae'') is a species of

gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

nitrogen-fixing Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

rod-shaped Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. They can be free-living or symbionts of leguminous plants in

root nodules Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known a ...

. ''E.medicae'' was first isolated from root nodules on plants in the genus ''

Medicago ''Medicago'' is a genus of flowering plants, commonly known as medick or burclover, in the legume family (Fabaceae). It contains at least 87 species and is distributed mainly around the Mediterranean Basin, and extending across temperate Eurasia ...

''. Some strains of ''E.medicae'', like WSM419, are aerobic. They are chemoorganotrophic

mesophiles A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37 °C (about 99 °F). The term is mainly applied ...

that prefer temperatures around 28 °C. In addition to their primary genome, these organisms also have three known plasmids, sized 1,570,951 bp, 1,245,408 bp and 219,313 bp. Colonies of ''Ensifer medicae'' are mucoid and ring-shaped and can be viewe
here

Phylogeny/Taxonomy

The genus is sometimes referred to as ''Sinorhizobium'' instead of ''Ensifer'' (see explanation at the genus article). Two major subgroups include ''E.medicae'' strain A321 and ''E. medicae'' strain WSM419
This
phylogenetic tree shows relatedness to the rest of the ''Rhizobiaceae'' family based on 16s rRNA gene sequences.

Ecological role and symbiosis with legumes

This bacterium is often involved in mutualistic relationships with legumes. It performs atmospheric

nitrogen fixation Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

for the plants and in exchange it receives organic carbon through the process of rhizodeposition. Free-living bacteria become housed inside specialized root cells in

, which creates anaerobic microhabitat in which efficient N-fixation can occur. This mutualism has been observed with many plant species, including ''

Medicago polymorpha ''Medicago polymorpha'' is a plant species of the genus ''Medicago''. It is native to the Mediterranean basin but is found throughout the world. It forms a symbiotic relationship with the bacterium ''Sinorhizobium medicae'', which is capable of n ...

'' and ''

Medicago truncatula ''Medicago truncatula'', the barrelclover, strong-spined medick, barrel medic, or barrel medick, is a small annual legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant tall with trif ...

'' plants from around the world. The extent of the mutualism may be dependent upon

soil pH Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...

, as it was thought that acidity can constrain ''Sinorhizobium medicae''. Current thinking is that acid resistance, particularly in strain WSM419, can be transferred on plasmids.

Agricultural Importance

Because ''E. medicae'' associates well with plants in the genus ''Medicago'' and increases plants growth, crop fields are commonly inoculated with the bacteria in addition to or in replacement of synthetic fertilizers. Specialized strains are developed for challenging conditions. The SRDI554 strain is highly halotolerant, making it an ideal inoculum for the salt-tolerant pasture legume ''

Melilotus siculus ''Melilotus'', known as melilot or sweet clover is a genus of legumes in the family Fabaceae, native to Europe, Asia, and Africa. The genus is closely related to ''Trifolium'' (clovers). Several species are common grassland plants and weeds of ...

'' (messina). The WSM1115 is, on the other hand, acid-tolerant.

References

{{Taxonbar, from=Q21326825 Rhizobiaceae Bacteria described in 1996