Members of the genus Selenomonas (motile crescent-shaped bacteria in general) are referred to trivially as selenomonads. The genus Selenomonas constitutes a group of motile crescent-shaped bacteria and includes species living in the gastrointestinal tracts of animals, in particular the ruminants. A number of smaller forms discovered with the light microscope are now in culture but many, especially the large selenomonads are not, owing to their fastidious and incompletely known growth requirements.
Gram stain
The family Veillonellaceae was transferred from the order Eubacteriales to the new order Selenomonadales in the new class Negativicutes. Despite most of the members of the Bacillota 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.
This transfer now appears to have been mistaken. On further examination the Selenomonads appear to be members of the Clostridia.
Etymology
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. 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 certain 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.
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, 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. These crescents live only a short time under the light 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, which was first described by Woodcock & LaPage, studied later by Lessel & Breed (with photomicrographic addendum from C. F. Robinow), then by Jeynes, who (mistakenly) interpreted it as a "blepharoplast".