Sphingomonas was defined in 1990 as a group of Gram-negative, rod-shaped, chemoheterotrophic, strictly aerobic bacteria. They possess ubiquinone 10 as their major respiratory quinone, contain glycosphingolipids (GSLs), specifically ceramide, instead of lipopolysaccharide (LPS) in their cell envelopes, and typically produce yellow-pigmented colonies. The GSL serves to protect the bacteria from antibacterial substances. Unlike most Gram-negative bacteria, Sphingomonas carries endotoxins and has a hydrophobic surface characterized by the short nature of the GSL's carbohydrate portion. 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, Sphingomonas was subdivided into different genera: Sphingomonas, Sphingobium, Novosphingobium, Sphingosinicella, and Sphingopyxis. These genera are commonly referred to collectively as sphingomonads. Distinct from other sphingomonads, Sphingomonas genomic structure includes a unique lipid formation, major 2-OH fatty acids, homospermidine as the primary polyamine, and signature nucleotide bases within the 16S rRNA gene. The bacteria holds 3914 proteins, 70 organizational RNA, and 3,948,000 base pairs (incomplete observation). Habitat The sphingomonads are widely distributed in nature, having been isolated from many different land and water habitats, as well as from plant root systems, clinical specimens, and other sources; this is due to their ability to survive in low concentrations of nutrients, as well as to metabolize a wide variety of carbon sources. Numerous strains have been isolated from environments contaminated with toxic compounds, where they display the ability to use the contaminants as nutrients. Role in disease 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. In contrast, the seed-endophytic strain Sphingomonas melonis ZJ26 that can be naturally enriched in certain rice cultivars, confers diseases resistance against a bacterial pathogen and is vertically transmitted among plant generations via their seeds. Applications Biotechnological utilization Due to their biodegradative and biosynthetic capabilities, sphingomonads have been used for a wide range of biotechnological applications, from bioremediation of environmental contaminants to production of extracellular polymers such as sphingans (e.g., gellan, welan, and rhamsan) used extensively in the food and other industries. The shorter carbohydrate moiety of GSL compared to that of LPS results in the cell surface being more hydrophobic than that of other Gram-negative bacteria, probably accounting for both Sphingomonas''' sensitivity to hydrophobic antibiotics and its ability to degrade hydrophobic polycyclic aromatic hydrocarbons. One