TERM PAPER

SUBJECT NAME:MICROBIAL PHYSIOLOGY AND METABOLISM

SUBJECT CODE: BTY538

TITLE:INDUSTRIAL IMPORTANCE OF ZYMOMONAS MOBILIS

SUBMITTED BY:

SHASHI SHARMA

SECTION:P8003

ROLL NO:B 15

REG. NO:11006142

CONTENTS:

1) INTRODUCTION

2) INDUSTRIAL IMPORTANCE

3) DRAWBACK

4) RECENT RESEARCH

INTRODUCTION:

Zymomonas mobilis is a bacterium belonging to the genus Zymomonas. It is notable for its 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 in European countries.

Z. mobilis degrades sugars to pyruvate using the Entner-Doudoroff pathway. The pyruvate is then fermentated to produce ethanol and carbon dioxide as the only products.

The advantages of Z. mobilis over S. cerevisiae with respect to producing bioethanol:

higher sugar uptake and ethanol yield, lower biomass production, higher ethanol tolerance, does not require controlled addition of oxygen during the

fermentation, amenability to genetic manipulations.

However, it has a severe limitation compared to yeast: its utilizable substrate range is restricted to glucose, fructose, and sucrose. Using biotechnological methods, scientists are currently trying to overcome this. A variant of Z. mobilis that is able to use certain pentoses as a carbon source has been developed.

An interesting characteristic of Z. mobilis is that its plasma membrane contains hopanoids, pentacyclic compounds similar to eukaryotic sterols. This allows it to have an extraordinary tolerance to ethanol in its environment, around 13%.

. Zymomonas mobilis Scientific classification Kingdom: BacteriaPhylum: ProteobacteriaClass: Alpha ProteobacteriaOrder: SphingomonadalesFamily: SphingomonadaceaeGenus: ZymomonasSpecies: Z. mobilisBinomial name Zymomonas mobilis

The natural habitat of this organism includes sugar-rich plant saps where the bacterium ferments sugar to ethanol. The high conversion of sugars to ethanol makes this organism useful in industrial production systems, particularly in production of bioethanol for fuel. A recombinant strain of this bacterium is utilized for the conversion of sugars, particularly xylose, which is not utilized by another common sugar-fermenting organism such as yeast, to ethanol. Since xylose is a common breakdown product of cellulose or a waste component of the agricultural industry, it is an attractive source for ethanol production. Zymomonas mobilis was chosen for this process as it is ethanol-tolerant (up to 120 grams of ethanol per litre) and productive (5-10% more ethanol than Saccharomyces). This bacterium ferments using the Enter-Doudoroff pathway, with the result that less carbon is used in cellular biomass production and more ends up as ethanol, another factor that favors this organism for ethanol production.

Zymomonas mobilis subsp. mobilis strain ZM4. This strain was isolated from fermenting sugarcane juice and is wild-type and unable to utilize xylose.

Cellular features Environment Temperature

Gram

stain

Shape

Arrangement

Endospores

Motility

Salinity

Oxygen Req.

Habitat

Opt. temp.

Range

- Rod Pairs No Anaerobic

Multiple

25-30

Mesophilic

Pathogenic in: No

RECENT RESEARCH: 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 .

INDUSTRIAL IMPORTANCE:

The bacterium Zymomonas mobilis is a highly potent ethanol producer already used in industrial-scale fermentations. Zymomonas converts sugars to ethanol and carbon dioxide, exhibiting up to 98% yields, minimal by-products, simple fermentation requirements, and several-fold the production rates of yeast. Beside its robust capability for biofuel production, this organism has been additionally proposed as a desirable candidate for sorbitol, levan, and other fine-chemical production, and it has attracted interest for its high content of membrane steroids, called hopanoids.

Zymomonas mobilis, a gram-negative bacterium, is considered as an alternative organism in large-scale fuel ethanol production. Comparative laboratory- and pilot-scale studies on kinetics of batch fermentation of Z. mobilis versus a variety of yeast have indicated the suitability of Z. mobilis over yeasts due to the following advantages:

 

i. its higher sugar uptake and ethanol yield,ii. its lower biomass production,

iii. its higher ethanol tolerance,iv. it does not require controlled addition of oxygen during the

fermentation, andv. its amenability to genetic manipulations.

Zymomonas mobilis are better candidates for industrial alcohol production. Z. mobilis possesses advantages over S. cerevisiae with respect to ethanol productivity and tolerance, thus encouraging researchers for exploiting Z. mobilis ability to utilize sucrose, glucose, and fructose by Entner–Deudoroff pathway. The bottlenecks in Z. mobilis are:

(i) its inability to convert complex carbohydrate polymers like cellulose, hemicellulose, and starch to ethanol,

(ii) its resulting in byproducts such as sorbitol, acetoin, glycerol, and acetic acid, and

(iii) formation of extracellular levan polymer. To circumvent these problems, 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 into Z. mobilis.

DRAWBACK:

The only limitation of Z. mobilis compared to the yeast is that its utilizable substrate range is restricted to glucose, fructose, and sucrose.

Z. mobilis was originally isolated from alcoholic beverages like the African palm wine, the Mexican ‘pulque’, and also as a contaminant of cider and beer in European countries

. On the basis of evaluation using the modern taxonomic approaches, the genus Zymomonas has only one species with two subspecies, Z. mobilis subsp. mobilis and Z. mobilis subsp. pomaceae. The ability to utilize sucrose as a carbon source distinguishes Z. mobilis from Z. anaerobia. It is one of the few facultative anaerobic bacteria which metabolizes glucose and fructose via the Entner–Deudoroff (E–D) pathway, which is usually present in aerobic microorganisms.

Under anaerobic conditions, Z. mobilis produces byproducts such as acetoin, glycerol, acetate, and lactate, which result in reduced production of ethanol from glucose. During growth of Z. mobilis in fructose, the formation of ace-toin, acetic acid, and acetaldehyde was clearly more pronounced than when grown in glucose. However the cell yield was low during its growth in fructose.

REFRENCES:

1. Swings, J. and De Ley, J., in Bergies Manual of Systematic Bacteriology (eds Krieg, N. R. and Holt, J. S.), William and Wilkins, Baltimore, USA, 1984, vol. 2, pp. 576.

2. Parker, C., Barnell, W. O., Snoep, J. L., Ingram, L. O. and Conway, T., Mol. Microbiol., 1995, 15, 795–802.

3. Toran-Diaz, I., Jain, V. K., Allais, J. J. and Baratti, J., Biotechnol. Lett., 1985, 7, 527–530.

4. Scott, C. D., Davison, B. H., Scott, T. C., Woodward, J., Dees, C. and Rothrock, D. S., Appl. Biochem. Biotechnol., 1994, 45/46, 641–653.