The genus
Selenomonas constitutes a group of motile crescent-shaped bacteria within the
VeillonellaceaeThe Veillonellaceae are a family of the Clostridia, formerly known as Acidaminococcaceae. Bacteria in this familiy are grouped together mainly based on genetic studies, which place them among the Firmicutes. Supporting this placement, several species are capable of forming endospores. However,...
family and include
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
living in the
gastrointestinal tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
s of
animalAnimals are a major group of mostly 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...
s, in particular, the Ruminants.
A few of the smaller forms discovered with the light
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
are now in culture but many are not because of their
fastidiousA 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.
The name Selenomonas simply refers to the crescent moon-shaped profile of this organism and not to any dependence on the element selenium.
The genus
Selenomonas constitutes a group of motile crescent-shaped bacteria within the
VeillonellaceaeThe Veillonellaceae are a family of the Clostridia, formerly known as Acidaminococcaceae. Bacteria in this familiy are grouped together mainly based on genetic studies, which place them among the Firmicutes. Supporting this placement, several species are capable of forming endospores. However,...
family and include
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
living in the
gastrointestinal tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
s of
animalAnimals are a major group of mostly 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...
s, in particular, the Ruminants.
A few of the smaller forms discovered with the light
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
are now in culture but many are not because of their
fastidiousA 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.
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 .
During more recent years the organism has been variously described as:
- Ancyromonas ruminantium ,
- Selenomastix ruminantium ,
- Spirillum ruminantium ,
- Selenomonas ruminantium ,.
As can be ascertained from the above
nomenclatureThe 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 Selenomonad group are undoubtedly the large motile
crescentIn 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 is...
s found in the warm anaerobic nutrient-rich
microecosystemMicroecosystems 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
ruminantPhysiologically, a ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first stomach, known as the rumen, then regurgitating the semi-digested mass, now known as cud, and chewing it again. The process of rechewing the cud to...
rumenThe 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
gingivaThe gingiva , or gums, consists of the mucosal tissue that lies over the alveolar bone.-General description:Gingiva are part of the soft tissue lining of the mouth. They surround the teeth and provide a seal around them...
(
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, 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
transmission electron microscopyTransmission 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 "
fascicleA fascicle is a bundle or a cluster.Fascicle may also refer to:In anatomy:* Muscle fascicle, a bundle of skeletal muscle fibers surrounded by connective tissue* Nerve fascicle,a larger bundle of axons enclosed by the perineurium...
" 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 membraneIn 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, 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.