Selenomonad

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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. 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 unusual cell form (with its in-folding of the cell membrane behind the flagella) results in bilateral symmetry - an unusual property for prokaryotes.

The literature on Selenomonas has roots dating back to the 19th century:

• Ancyromonas ruminantium (Certes 1889),
• Selenomastix ruminantium (Woodcock & Lapage 1913-14),
• Spirillum ruminantium (Macdonald et al. 1959),
• Selenomonas ruminantium (Wenyon 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 interesting members of the Selenomonad group 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). 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 again preparations of native rumen contents were examined for the first time by electron microscopy of thin sections, negative stains and freeze-fracture replicas.^{[2] [3]} 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. Successful attempts to maintain them in continuous culture over short terms have been reported (e.g. by Prins 1971), but long term culturing has not been possible so far. Genetic sequencing of the large crescents will no doubt provide the essential information needed to better understand these fascinating organisms.

Sketch of a sagittal longitudinal ultrathin section through an unidentified large selenomonad from sheep rumen liquor (which seems to correspond closely to descriptions of S. palpitans).

[edit] References

1. ^ Nomenclature of genus selenomonas on DSMZ
2. ^ Chalcroft J P, Bullivant S, Howard B H (1973). "Ultrastructural studies on Selenomonas ruminantium from the sheep rumen". Journal of General Microbiology 79 (1): 135–146. PMID 4773919. 
3. ^ Kingsley V V, Hoeniger J F M (1973). "Growth, Structure and Classification of Selenomonas". Bacteriological Reviews 37 (4): 479–521. PMID 4129090. 

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