A Microbial Biorealm page on the genus Zymomonas mobilis
Higher order taxa
Bacteria; Proteobacteria; Alpha Proteobacteria; Sphingomonadales; Sphingomonadaceae
Description and significance
Zymomonas mobilis is a rod shaped gram-negative bacterium that can be found in sugar rich plant saps. It is usually 2-6 μm long and 1-1.4 μm wide, but this can vary significantly. In high CO2 or ethanol concentrations slime and granular layers have been seen around the cell. It has been isolated from sugar cane as well as alcoholic beverages such as African palm wine; it is also known to cause cider sickness and spoiling of beer. However, the spoiling of beer is very limited due to its optimal temperature range of 25-30°C. It is being studied for its ability to ferment sugar to ethanol. Its ability to efficiently ferment carbohydrates using the Enter-Doudoroff pathway makes it an attractive candidate for producing bioethanol for fuel. It has also shown a high growth rate, tolerance to ethanol as well as being amendable to engineering  .
There is currently one compete genome sequence and one in progress for the NCBI genome list for Zymomonas mobilis. The genome has a circular chromosome with 1,998 open reading frames, three ribosomal RNA transcription units and is made up of 2,056,416 base pairs. It has an overall G+C content of 46.3%. It appears to only be able to metabolize glucose through the Entner-Doudoroff pathway because the genome does not have recognized genes for 6-phosphofructokinase, an enzyme essential for the Embden-Meyerhof-Parnas pathway. It also lacks the genes for the enzymes 2-oxoglutarate dehydrogenase complex and malate dehydrogenase both of which are involved in the tricarboxylic acid cycle .
Cell structure and metabolism
Zymomonas mobilis is a gram-negative bacteria. It is rod-shaped and not mobile. Z. mobilis has hopanoids in its plasma membrane and this is thought to be a key to its high ethanol tolerance. Its plasma membrane has a phospholipid composition with a major component being phosphatidylethanolamine and lesser amounts of phosphatidylglycerol, cardiolipin, dimethyl phosptidylethanolamine and phosphatidylcholine. In environments with increased ethanol and glucose, there is a decrease in phosphatidylethanolamine and phosphatidylglycerol and an increase in phosphatidylcholine .
It uses the Enter-Doudoroff pathway for fermentation, making it very efficient and able to produce ethanol near theoretical levels. It is unable to use other pathways to obtain energy as it appears to lack key enzymes for the Embden-Meyerhof-Parnas pathway and the tricarboxylic acid cycle. In its naturally occurring form it can only metabolize glucose, fructose and sucrose to ethanol; it is unable to ferment more complex carbohydrates. However, engineered strains have been produced that are capable of metabolizing complex carbohydrates and this is an area that is being heavily researched . Sucrose can cause problems for the efficiency of fermentation because of byproducts which are produced such as levan and sorbitol and this greatly increases the biomass limiting ethanol production . Ethanol production inhibition may also be caused by the presence of acetic, formic or propionic acids as well as higher concentrations of oxygen and carbon dioxide. However, ethanol itself seems to be the biggest inhibitor because of its affect on the plasma membrane; it causes the membrane to become more permeable allowing some cofactors and coenzymes from the Entner-Douduroff pathway to leave .
Application to Biotechnology
Z. mobilis’ ability to efficiently produce ethanol is of particular interest. This is particularly true due to the growing need for renewable energy sources and bioethanol ranks among the top choices at this time. Researchers are working on strains that are capable of fermenting more complex carbohydrates that are widely available such as cellulose and molasses . And in terms of organisms being studied for ethanol production Z. mobilis is a leading candidate because of its high level of efficiency, high level of ethanol tolerance and its ability to be genetically altered .
Also, the byproduct of sucrose fermentation levan can be a useful product. Levan is used in food and pharmaceuticals as a thickening, gelling and suspending agent .
Researchers are investigating ethanol production from cellulosic materials by genetically engineered Zymomonas mobilis. Because of its inability to naturally metabolize complex carbohydrates strains are being engineered with the purpose of being able to ferment complex carbohydrates. In this study a β-glucosidase gene from Ruminococcus albus was introduced to Z. mobilis in an attempt to produce ethanol from cellulose. Using a tag on the N-terminus of the 53-amino acid protein they were able to transport 61% of the β-glucosidase gene activity. This resulted in a production of .49g ethanol/g cellobiose .
Fermentation of molasses by Zymomonas mobilis: Effects of temperature and sugar concentration on ethanol production. In this study scientists looked at the effects of temperature and molasses concentration on ethanol production. They used factorial design so they could study varied conditions concurrently; the different conditions were varying combinations of temperature, molasses concentration and culture times. They concluded that the optimal conditions found for ethanol production were 200g/L of molasses at 30°C for 48 hours and this produced 55.8g ethanol/L .
Over-expression of xylulokinase in a xylose-metabolising recombinant strain of Zymomonas mobilis It has been theorized that xylulokinase is the rate-limiting enzyme in xylose-metabolism in the Z. mobilis recombinant Zm4/AcR. To test this a plasmid was introduced which caused over expression of xylulokinase. This resulted in a 3-fold increase in xylulokinase expression. However, there was no increase in the xylose metabolism. So, based on this study xylulokinase does not appear to be the rate-limiting enzyme for the xylose-metabolising recombinant of Z. mobilis .
Also, it has been announced that a Dupont/Broin partnership will look to develop a recombinant Z. mobilis for commercial manufacturing of ethanol from lignocellulosic residues .
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Edited by David Ly student of Rachel Larsen and Kit Pogliano