Selenomonad
Encyclopedia
The genus Selenomonas constitutes a group of motile crescent-shaped bacteria within the Veillonellaceae family and includes species
living in the gastrointestinal tract
s of animal
s, 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.
from the order Clostridiales to the new order Selenomonadales in the new class Negativicutes
. 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.
and the noun monas
(μόνας) which in microbiology has come to mean bacterium.
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.
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 crescent
s 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.
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. 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.
and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
living in the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
s of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s, in particular, the Ruminants.
A few of the smaller forms discovered with the light microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
are now in culture but many are not because of their fastidious
Growth medium
A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
and incompletely known requirements.
Gram stain
The family Veillonellaceae was transferredBacterial taxonomy
Bacterial taxonomy is the taxonomy, i.e. the rank-based classification, of bacteria.In the scientific classification established by Carl von Linné, each species has to be assigned to a genus , which in turn is a lower level of a hierarchy of ranks .In the currently accepted classification...
from the order Clostridiales to the new order Selenomonadales in the new class Negativicutes
Negativicutes
The Negativicutes is a class of firmicute bacteria, whose members have a peculiar cell wall composition which stains Gram negative, unlike most other members of the Firmicutes. The family Veillonellaceae, formerly known as Acidaminococcaceae, we once considered members of the clsss Clostridia. The...
. Despite most of the members of the Firmicutes
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
staining positive for the Gram stain and being trivially
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
called "low-GC Gram-positives" (c.f. Bacterial phyla
Bacterial phyla
The bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...
),members of the Negativicutes
Negativicutes
The Negativicutes is a class of firmicute bacteria, whose members have a peculiar cell wall composition which stains Gram negative, unlike most other members of the Firmicutes. The family Veillonellaceae, formerly known as Acidaminococcaceae, we once considered members of the clsss Clostridia. The...
stain Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
and possess a double bilayer.
Etymology
The etymology of the name Selenomonas comes from the Ancient Greek noun selênê (σελήνη), meaning the moon, a linking -o-Bacterial taxonomy
Bacterial taxonomy is the taxonomy, i.e. the rank-based classification, of bacteria.In the scientific classification established by Carl von Linné, each species has to be assigned to a genus , which in turn is a lower level of a hierarchy of ranks .In the currently accepted classification...
and the noun monas
-monas
The suffix -monas is used in microbiology for many genera and is intended to mean "unicellular organism".-Meaning:The suffix -monas is used in microbiology for many genera in a similar way to -bacter, -bacillus, -coccus or -spirillum...
(μόνας) which in microbiology has come to mean bacterium.
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.
Trivial name
Members of the genus Selenomonas (and motile crescent-shaped bacteria in general) are referred to trivially as selenomonads.History and Description
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.During more recent years the crescent-shaped organism observed in ruminant stomachs has been variously described as:
- Ancyromonas ruminantium,
- Selenomastix ruminantium,
- Spirillum ruminantium,
- Selenomonas ruminantium ,.
As can be ascertained from the above nomenclature
International Code of Nomenclature of Bacteria
The International Code of Nomenclature of Bacteria or Bacteriological Code governs the scientific names for bacteria, including Archaea. It denotes the rules for naming taxa of bacteria, according to their relative rank...
, 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 crescent
Crescent
In art and symbolism, a crescent is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points .In astronomy, a crescent...
s found in the warm anaerobic nutrient-rich microecosystem
Microecosystem
Microecosystems can exist in locations which are precisely defined by critical environmental factors within small or tiny spaces.Such factors may include temperature, pH, chemical milieu, nutrient supply, presence of symbionts or solid substrates, gaseous atmosphere etc.-Pond microecosystems:These...
provided by ruminant
Ruminant
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...
rumen
Rumen
The rumen, also known as a paunch, forms the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed...
, guinea-pig caecum (S. palpitans) and even pockets in the human gingiva
Gingiva
The gingiva , or gums, consists of the mucosal tissue that lies over the mandible and maxilla inside the mouth.-General description:...
(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.
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".
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
of thin sections, negative stains and freeze-fracture replicas. 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
Polar membrane
In the scientific literature polar membrane has acquired two separate meanings.1) A lipid biomembrane expressing polarity in the electrical sense...
", 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.
Phylogeny
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LSPN)and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project