Selenomonas

The genus Selenomonas constitutes a group of motile crescent-shaped bacteria within the Veillonellaceae family 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. The family Veillonellaceae was transferred 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. This transfer now appears to have been mistaken. On further examination the Selenomonads appear to be members of the Clostridia. 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. Members of the genus Selenomonas (and motile crescent-shaped bacteria in general) are referred to trivially as selenomonads. 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: 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.