Ethanol fermentation technology - Zymomonas mobilis

Due to dwindling of fossil fuel, microbial production of bio-fuel from orga nic byproducts has acquired significance in recent years. Ethanol has been trusted as an alternate fuel for the future. Even though several microorgan isms, including Clostridium sp., have been as ethanologenic microbes, the y east Saccharomyces cerevisiae and facultative bacterium Zymomonas mobilis a re better candidates for indus trial alcohol production. Z. mobilis possess es advantages over S. cerevisiae with respect to ethanol productivity and t olerance, thus encouraging researchers for exploiting Z. mobilis ability to utilize sucrose, glucose, and fructose by Entner-Deudoroff pathway, bottle necks in Z. mobilis are: (i) its inability to convert complex carbohydrate polymers like cellulose, hemicellulose, and starch to ethanol, (ii) its res ulting in byproducts such as sorbitol, acetoin, glycerol, and acetic acid, and (iii) formation of extracellular levan polymer. To circumvent these pro blems, genetic manipulation of Z. mobilis has been attempted for broadening the utilizable range of Z. mobilis, i.e. genes encoding several hydrolytic enzymes from related bacterial species have been cloned, and transferred i nto Z. mobilis. Interestingly, a pet operon (production of ethanol) was con structed by combining pde (pyruvate decarboxylase) and adhII (alcohol dehyd rostrains genase) genes of Z. mobilis, and transferred to other bacterial s trains to make them ethanologenic novel strains. Through classical mutation and selection approaches, mutants of Z. mobilis with improved fermentation characteristics and without byproduct formation have been obtained. In add ition to ethanol, Z. mobilis has also been metabolically engineered to prod uce L-alanine and L-lactic acid. Genes encoding beta-carotene synthesis hav e also been cloned and successfully expressed in Z. mobilis to enrich the f ermented nutrients of farm animals. Several applications of levan in food a nd pharmaceutical industries provide an opportunity to exploit Z. mobilis f or large scale production of levan. The merits of Z. mobilis suggest the po tential use of this organism in industrial production of various fermentati on products.