Zymomonas
| Zymomonas mobilis | |
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| Scientific classification | |
| Domain: | Bacteria |
| Phylum: | Proteobacteria |
| Class: | Alphaproteobacteria |
| Order: | Sphingomonadales |
| Family: | Sphingomonadaceae |
| Genus: | Zymomonas |
| Species: | Z. mobilis |
| Binomial name | |
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Zymomonas mobilis (Lindner 1928) De Ley and Swings 1976 |
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Zymomonas mobilis is a Gram negative, facultative anaerobic, non-sporulating, polarly-flagellated, rod-shaped bacterium. It is the only species found in the genus Zymomonas. It has notable bioethanol-producing capabilities, which surpass yeast in some aspects. It was originally isolated from alcoholic beverages like the African palm wine, the Mexican pulque, and also as a contaminant of cider and beer (cider sickness and beer spoilage) in European countries.
Zymomonas is an unwanted waterborne bacteria in beer, creating an estery-sulfury flavor due to the production of acetaldehyde and hydrogen sulfide. This can be likened to a rotten apple smell or fruity odor. Zymomonas have not been reported in lager breweries due to the low temperatures (8–12 °C) and stringent carbohydrate requirements (able to ferment only sucrose, glucose, and fructose). It is commonly found in cask-conditioned ales where priming sugar is used to carbonate the beer. The optimum growth temperature is 25 to 30 degrees Celsius.
Zymomonas mobilis degrades sugars to pyruvate using the Entner-Doudoroff pathway. The pyruvate is then fermented to produce ethanol and carbon dioxide as the only products (analogous to yeast).
The advantages of Z. mobilis over S. cerevisiae with respect to producing bioethanol:
However, in spite of these attractive advantages, several factors prevent the commercial usage of Z. mobilis in cellulosic ethanol production. The foremost hurdle is that its substrate range is limited to glucose, fructose and sucrose. Wild-type Z. mobilis cannot ferment C5 sugars like xylose and arabinose which are important components of lignocellulosic hydrolysates. Unlike E. coli and yeast, Z. mobilis cannot tolerate toxic inhibitors present in lignocellulosic hydrolysates such as acetic acid and various phenolic compounds. Concentration of acetic acid in lignocellulosic hydrolysates can be as high as 1.5% (w/v), which is well above the tolerance threshold of Z. mobilis.
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