Gellan gum is a water-soluble anionic polysaccharide produced by the bacterium Sphingomonas elodea (formerly Pseudomonas elodea based on the taxonomic classification at the time of its discovery). The gellan-producing bacterium was discovered and isolated by the former Kelco Division of Merck & Company, Inc. in 1978 from the lily plant tissue from a natural pond in Pennsylvania. It was initially identified as a substitute gelling agent at significantly lower use level to replace agar in solid culture media for the growth of various microorganisms. Its initial commercial product with the trademark as Gelrite gellan gum, was subsequently identified as a suitable agar substitute as gelling agent in various clinical bacteriological media. Chemical structure The repeating unit of the polymer is a tetrasaccharide, which consists of two residues of D-glucose and one of each residues of L-rhamnose and D-glucuronic acid. The tetrasaccharide repeat has the following structure:[D-Glc(β1→4)D-GlcA(β1→4)D-Glc(β1→4)L-Rha(α1→3)]n Gellan gum products are generally put into two categories, low acyl and high acyl depending on number of acetate groups attached to the polymer. The low acyl gellan gum products form firm, non-elastic, brittle gels, whereas the high acyl gellan gum forms soft and elastic gels. Microbiological gelling agent Gellan gum is initially used as a gelling agent, alternative to agar, in microbiological culture. It is able to withstand 120 °C heat. It was identified as an especially useful gelling agent in culturing thermophilic microorganisms. One needs only approximately half the amount of gellan gum as agar to reach an equivalent gel strength, though the exact texture and quality depends on the concentration of the divalent cations present. Gellan gum is also used as gelling agent in plant cell culture on Petri dishes, as it provides a very clear gel, facilitating light microscopical analyses of the cells and tissues. Although advertised as being inert, experiments with the moss Physcomitrella patens have shown that choice of the gelling agent—agar or Gelrite—does influence phytohormone sensitivity of the plant cell culture. Food science As a food additive, gellan gum was first approved for food use in Japan (1988). Gellan gum has subsequently been approved for food, non-food, cosmetic and pharmaceutical uses by many other countries such as US, Canada, China, Korea and the European Union etc. It is widely used as a thickener, emulsifier, and stabilizer. It has E number E418. It was an integral part of the now defunct Orbitz soft drink. It is used as