Sphingomonas and Related Genera
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Sphingomonas and Related Genera David L. BALKWILL, J. K. FREDRICKSON AND M. F. ROMINE
Introduction The genus Sphingomonas was defined by Yabuuchi et al. (1990) as a group of Gramnegative, rod-shaped, chemoheterotrophic, strictly aerobic bacteria that possess ubiquinone 10 as the major respiratory quinone, contain glycosphingolipids (GSLs) instead of lipopolysaccharide in their cell envelopes, and typically produce yellow-pigmented colonies. By 2001, the genus included more than 20 species that were quite diverse in terms of their phylogenetic, ecological, and physiological properties. As a result, Takeuchi et al. (2001) subdivided Sphingomonas into four genera: Sphingomonas, Sphingobium, Novosphingobium and Sphingopyxis. These genera are referred to collectively as “sphingomonads” in this chapter. The sphingomonads are widely distributed in nature, having been isolated from many different aqueous and terrestrial habitats, as well as from plant root systems, clinical specimens, and other sources. Sphingomonads are metabolically versatile and, thus, are able to utilize a wide range of naturally occurring organic compounds as well as many types of refractory environmental contaminants. Because of this, there has been much interest in the metabolic pathways of these organisms, the properties of the enzymes involved with those pathways, and the genetics of their catabolic processes. Some of the sphingomonads (especially Sphingomonas paucimobilis) also play a role in human disease, primarily by causing a range of mostly nosocomial, non-life-threatening infections that typically are easily treated by antibiotic therapy. Owing to their remarkable biodegradative and biosynthetic capabilities, sphingomonads have been utilized for a wide range of biotechnological applications, from bioremediation of environmental contaminants to production of extracellular polymers such as sphingans used extensively in the food and other industries.
Taxonomy and Phylogeny The genus Sphingomonas was defined by Yabuuchi et al. (1990) as a group of Gram-
negative, rod-shaped, chemoheterotrophic, strictly aerobic bacteria that possess ubiquinone 10 as the major respiratory quinone, contain glycosphingolipids (GSLs) in their cell envelopes, and typically produce yellow-pigmented colonies. Other characteristics, such as presence of certain lipids and the utilization of various substrates, were also included in the definition of the genus. However, it was the presence of GSLs that most clearly distinguished Sphingomonas from other members of the a-subclass of the Proteobacteria (Woese, 1987; Stackebrandt et al., 1988) at that time. Yabuuchi et al. (1990) characterized 16 strains of bacteria in the Japan Collection of Microorganisms (JCM) that had been received as Pseudomonas paucimobilis (including the type strain, JCM 7516T; Holmes et al., 1977), Xanthomonas sp., Flavobacterium sp., Flavobacterium devorans, Flavobacterium capsulatum (Leifson, 1962), or Sphingobacterium sp. On the basis of physiol
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