Selenomonad

Taxobox
color = lightgrey

name = Selenomonas
regnum = Bacteria
phylum = Firmicutes
classis = Clostridia
ordo = Clostridiales
familia = Acidaminococcaceae
genus = "Selenomonas"
genus_authority = Wenyon, 1926
subdivision_ranks = Species [ [http://www.dsmz.de/microorganisms/bacterial_nomenclature_info.php?genus=SELENOMONAS] ]
subdivision = "Selenomonas acidaminovorans"
"Selenomonas artemidis"
"Selenomonas dianae"
"Selenomonas flueggei"
"Selenomonas infelix"
"Selenomonas lacticifex"
"Selenomonas lipolytica"
"Selenomonas noxia"
"Selenomonas palpitans"
"Selenomonas ruminantium"
"Selenomonas sputigena"

The genus "Selenomonas" constitutes a group of motile crescent-shaped bacteria within the Acidaminococcaceae family and include 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.The name Selenomonas simply refers to the crescent moon-shaped profile of this organism and not to any dependence on the element selenium. The unique cell observed in the morphology of large Selenomonads (with its in-folding of the cell membrane behind the flagella) results in bilateral symmetry along the long axis - an unusual property for prokaryotes.

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 [cite book |author=Dobell, C. | title=Antony van Leeuwenhoek and his "little animals" | year=1932] .During more recent years the organism has been variously described as:
*"Ancyromonas ruminantium" [cite journal |author=Certes, A. | title=Note sur les micro-organismes de la panse des ruminants | journal=Bull. Soc. Zool. France |volume=14 |pages=70–73 | year=1889] ,
*"Selenomastix ruminantium" [cite journal |author=Woodcock, H. M. & G. Lapage | title=On a remarkable type of protistan parasite | journal=Quarterly Journal of Microscopical Science |volume=59 |pages=431–458 | year=1914] ,
*"Spirillum ruminantium" [cite journal |author=MacDonald, J. B. Madlener, E. M. & Socransky, S. S.| title=Observations on "Spirillum sputigenum" and its relationship to "Selenomonas" species with special reference to flagellation | journal=J. Bacteriol. |volume=77 |issue=5 |pages=559–565 | year=1959 |pmid=13654218 ] ,
*"Selenomonas ruminantium" [cite book |author=Wenyon, C.M. | title=Protozoology, Vol. 1 | year=1926] ,.

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 Selenomonad group are undoubtedly the large motile crescents found in the warm 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 [cite journal |author=Moir, R.J. & Masson, M.J. |title=An illustrated scheme for the microscopic identification of the rumen micro-organisms of sheep |journal=J. Pathol Bacteriol. |volume=64 |issue=2 |pages=343-350 |year=1952 |pmid=14946656] .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, the so-called "blepharoplast". These features were first described by Woodcock & LaPage in 1913, and later by Jeynes in 1955.

Years later, preparations of native rumen contents were examined for the first time by electron microscopy of thin sections, negative stains and freeze-fracture replicas. [cite journal |author=Chalcroft J.P, Bullivant S, & Howard B.H. |title=Ultrastructural studies on Selenomonas ruminantium from the sheep rumen | journal=Journal of General Microbiology |volume=79 |issue=1 |pages=135–146 |year=1973 |pmid=4773919 ] [cite journal |author=Kingsley V V, & Hoeniger J F M |title=Growth, Structure and Classification of Selenomonas | journal=Bacteriological Reviews |volume=37 |issue=4 |pages=479–521 |year=1973 |pmid=4129090 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=413832] 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 [cite journal |author=Prins, R. A. |title=Isolation, culture, and fermentation characteristics of Selenomonas ruminantium var. bryanti var. n. from the rumen of sheep. | journal=Journal of Bacteriology |volume=105 |issue=3 |pages=820–825 |year=1971 |pmid=4323298] , but long term culturing has not been possible so far. Genetic sequencing of the large crescents will hopefully provide the essential information needed to better understand amnd classify these fascinating organisms.

References