The Genus Selenomonas
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CHAPTER 1.3.3 E H T
A R E N E G
SA N M O N ELES
The Genus Selenomonas ROBERT B. HESPELL, BRUCE J. PASTER AND FLOYD E. DEWHIRST
Species of Selenomonas are defined as anaerobic, Gram-negative, curved or crescent-shaped rods that are motile by means of a tuft of flagella originating from the inner curvature of the cell. Selenomonads have been isolated from the rumen and ceca of mammals, and the human oral cavity. Depending upon the health or diet of the host, these bacteria can constitute a significant proportion of the total microbial population. In general, selenomonads are obligately saccharolytic, although some strains ferment lactate or amino acids. It has been suggested that the role of ruminal and intestinal selenomonads involves the fermentation of soluble sugars and lactate in their natural environments. Oral selenomonads may play a role in the pathogenesis of periodontal disease in humans. The first classification of selenomonads was by Miller (1887), who designated strains from the human mouth as Spirillum sputigenum, but the present classification system places these organisms in the genus Selenomonas as S. sputigena (Bryant, 1984; Johnson et al., 1985). As a result of recent findings, however, at least six species of oral selenomonad have been recognized (Moore et al., 1987). Phylogenetic studies based on 16S ribosomal RNA sequence analysis have shown that species of Selenomonas are closely related to the bacteria Centipeda periodontii, Pectinatus cerevisiiphilus, and Sporomusa paucivorans. The selenomonads are more distantly related to anaerobic Gram-negative cocci of the genera Veillonella and Megasphaera. Selenomonads, Veillonella and related bacteria comprise a phylogenetic grouping which is more closely related to Gram-positive bacteria than to typical Gram-negative bacteria.
Habitats Selenomonads have been observed in and isolated primarily from the rumen, the human mouth, and the cecum of mammals (Table 1). Gram-negative organisms having the cell morphology and flagellar arrangement typical for selenomonads have also been observed in river water (Leifson, 1960), and recently selenomonads have been isolated from anaerobic sewage This chapter was taken unchanged from the second edition.
sludge (Nanninga et al., 1987) and bog water (Harborth and Hanert, 1982). Selenomonads were probably first observed by Antonie van Leeuwenhoek in gingival scrapings from the human mouth (Dobell, 1960). Traditionally, speciation within Selenomonas has been based upon the habitat from which the strain was isolated (Buchanan and Gibbons, 1974; Lessel and Breed, 1954). Although helpful, this criterion has been largely supplanted by more adequate cytological, biochemical, and molecular criteria. Selenomonads appear to be part of the normal indigenous microflora of human gingival crevices and are often more abundant in those persons having clinically detectable gingivitis or peridontal disease. Oral selenomonads may play a role in peridontal disease, inasmuch as lipopolysaccharide purified from these bacterial species has been sho
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