Zymomonas

Zymomonas is a genus of bacteria. Members of this genus are gram negative, facultative anaerobic, non-sporulating, polarly-flagellated, rod-shaped bacteria.

The best-known species from this genus is Zymomonas mobilis. It is a fermenting bacteria that contributes to cider sickness and beer spoilage. It has a near theoretical ethanol yield from glucose, no oxygen requirement thus negating the need for expensive oxygen transfer, and high ethanol tolerance making it an effective microbe for ethanol biofuel production. 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.^[1] 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.

Several attempts have been made to engineer Z. mobilis to overcome its inherent deficiencies. National Renewable Energy Laboratory (NREL), USA has made significant contributions in expanding its substrate range to include C5 sugars like xylose and arabinose.^[2],[3] Acetic acid-resistant strains of Z. mobilis have been developed by rational metabolic engineering efforts, mutagenesis techniques ^[4] or adaptive mutation.^[5],[6] However, when these engineered strains metabolize mixed sugars in presence of inhibitors, the yield and productivity are much lower, thus preventing their industrial application.

An extensive adaptation process was used to improve xylose fermentation in Z. mobilis.^[5] Interestingly, by adapting a strain in a high concentration of xylose, significant alterations of metabolism occurred. One noticeable change was reduced levels of xylitol, a byproduct of xylose fermentation which can inhibit the strain's xylose metabolism. One of the reasons for lower xylitol production was mutation in a putative gene encoding for an aldo-keto reductase that catalyzes the reduction of xylose to xylitol.^[7],[8]

Beer spoilage

Zymomonas is a waterborne unwanted bacteria in beer, creating an estery-sulfury flavour 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 to 12 degrees Celsius) 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.

References

External links

• Ethanol fermentation technology – Zymomonas mobilis