| Selenomonas | |
|---|---|
| Scientific classification | |
| Kingdom: | Bacteria |
| Phylum: | Firmicutes |
| Class: | Negativicutes[1] |
| Order: | Selenomonadales[1] |
| Family: | Veillonellaceae |
| Genus: | Selenomonas Wenyon, 1926 |
| Species[2] | |
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Selenomonas acidaminovorans |
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The genus Selenomonas constitutes a group of motile crescent-shaped bacteria within the Veillonellaceae family and includes species living in the gastrointestinal tracts of animals, in particular, the Ruminants. A few of the smaller forms discovered with the light microscope are now in culture but many are not because of their fastidious and incompletely known requirements.
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The family Veillonellaceae was transferred from the order Clostridiales to the new order Selenomonadales in the new class Negativicutes.[1] Despite most of the members of the Firmicutes staining positive for the Gram stain and being trivially called "low-GC Gram-positives" (c.f. Bacterial phyla),members of the Negativicutes stain Gram-negative and possess a double bilayer.[1]
The etymology of the name Selenomonas comes from the Ancient Greek noun selênê (σελήνη), meaning the moon, a linking -o- and the noun monas (μόνας) which in microbiology has come to mean bacterium.[3] The name Selenomonas simply refers to the crescent moon-shaped profile of this organism and not in any way to the chemical element selenium. The unique cell morphology of the large selenomonads (with its in-folding of the cell membrane behind the flagella) would indicate bilateral symmetry along the long axis — an unusual property for prokaryotes.
Members of the genus Selenomonas (and motile crescent-shaped bacteria in general) are referred to trivially as selenomonads.
The literature on Selenomonas has roots dating back to the 19th century - and beyond - since the features and movements of living (then unclassified) crescent-shaped microorganisms from the human mouth were first described by Antonie van Leeuwenhoek in 1683.[4] During more recent years the crescent-shaped organism observed in ruminant stomachs has been variously described as:
As can be ascertained from the above nomenclature, the genus Selenomonas provides a fascinating history of scientific discovery, involving placement then re-placement in the classification systematics, oscillating between animal and bacterial kingdoms! In early descriptions it was thought to be a protozoan and hence for a while received the name Selenomastix.
The most morphologically interesting members of the selenomonads are undoubtedly the large motile crescents found in the warm anaerobic nutrient-rich microecosystem provided by ruminant rumen, guinea-pig caecum (S. palpitans) and even pockets in the human gingiva (S. sputigena). In the illustrated atlas of sheep rumen organisms of Moir and Masson their organisms nos. 4 and 5 represent two forms of the large Selenomonads.[9] These crescents live only a short time under the microscope but during that time display a remarkable "tumbling" motion produced by one (or two - during cell division) flagella emanating from a refractile basal body on the concave side, first described by Woodcock & LaPage in 1913, and later by Jeynes in 1955, who (mistakenly) interpreted it as a "blepharoplast".
Years later, preparations of native rumen contents were examined for the first time by transmission electron microscopy of thin sections, negative stains and freeze-fracture replicas.[10][11] and many of the reasons for previous confusion were clarified. The "flagellum" was found to be quite unrelated to the flagellum of ciliate protozoa, instead consisting of a "fascicle" of numerous bacterial-type flagella (each displaying 11-fold subunit symmetry), twisted just outside the cell body into helical bundles to form strong organs of propulsion. The large crescents (which are better described as "bean-shaped") have flagella which are quite differently inserted into the concave side of the cell from those of the smaller species of Selenomonas. The small selenomonads have a rather low number of individual flagella inserted in a longitudinal row along the concave side whereas the large selenomonads have a much larger number, inserted into a circular patch of the cell membrane in the concave side in a close-packed (hexagonal) pattern, each flagellum inserted into a bullet-shaped structure at the cell membrane. Another interesting feature is the refractile body behind the flagella. This is not related morphologically to the ciliate blepharoplast (a "9+2" centriole-related structure found in cryptogams, cycads, Ginkgo biloba and algae e.g. Euglena and Chlamydomonas). The structure in Selenomonas can perhaps best be described as a "basal sac" formed by special invagination (in-folding) of the "polar membrane" of the bacterial cell membrane in the middle of the concave side of the organism so that it lies directly behind the flagella. In other bacteria possessing this so-called "polar membrane", it is situated around the flagella insertion bases in the cell membrane, but never behind them in the cytoplasm. The large crescents, with their unique morphology, still present many puzzles in their systematics. It is already clear from ultrastructural features that the genus Selenomonas is most probably an artificial classification, bringing together possibly unrelated organisms, simply because of their common possession of crescent morphology and peculiar flagellar insertion location. Successful attempts to maintain the large crescents in continuous culture over short terms have been reported http://jdr.sagepub.com/content/88/6/519.full.pdf, but long term culturing has not been possible so far. Genetic sequencing of the large crescents should provide the essential information required to better understand and classify these fascinating organisms.
Research on obesity suggest that S. noxia may be an indicator of change in oral microbial ecology and could possibly or indirectly involve in obesity.[12]
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) [13] and National Center for Biotechnology Information (NCBI)[14] and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project [15]
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