Microbes

Microbes, the living Machine Speaker:-

Md. Mesbah Uddin Khulna University Bangladesh

Potential Microbes Bacteria

e.g. Zymomonas,

Clostridium, etc.

Fungi

e.g. Saccharomyces,

Kluyveromyces, etc.

 High sugarproducer consumption Cellulase

rate

Desirable Traits

High specific growth rate High salt tolerance (acetate) High volumetric productivity High Ethanol Tolerance for Bioethanol Production

High specific productivity Thermotolerance C5/C6 Cofermentation High shear tolerance Minimal nutrient requirements Entner-Doudoroff  Amenable to scale-up pathway Generally

Regarded as Safe (GRAS)

Available

gene transfer system

Yeast= Saccharomyces  

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Ethanol is inhibitory at high concentrations and the alcohol tolerance of yeast is critical for high yield. As the concentration increases, the growth rate is first reduced, whereas at higher concentrations the biosynthesis of ethanol itself is inhibited. Yeast is more sensitive to endogenously produced ethanol than to added from external sources to the fermentation system. Growth generally ceases at 5% ethanol ( v of ethanol in v of water=vv) and the production rate is reduced to zero at 6-10% (vv).

Zymomonas mobilis   

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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 (analogous to yeast).

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.

Comparison of Kinetic Parameter: Zymomonas vs. Saccharomyces  Ethanol

formation rate (g/g.h) 2.9 times higher  Growth rate (m) 2.4 times higher  Glucose uptake rate 2.6 times higher

Advantages of Zymomonas 

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The bacterium zy.. Has in recent years come under increasing study because it has a number of potential advantages: Osmotic tolerance to higher sugar concentration (up to 400 g/l) Relatively higher ethanol tolerance ( up to 130 g/l) Higher specific growth rate than yeast ( growth rate m of 0.27 compared to 0.13 for yeast; lab study) Carbohydrate metabolism by ED pathway, where only one mole of ATP is produced per mole of glucose used, thus reducing the amount of glucose that is converted to biomass rather than ethanol.

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Zymomonas mobilis • Advantages: – Natural fermentative microorganism (GRAS) – Near theoretical ethanol yield from glucose – Reduced yield loss to biomass formation – No oxygen requirement – Tolerant to inhibitors in hydrolysates – High ethanol tolerance – Fermentation at low pH – Grows at high sugar concentrations – High specific productivity • Limitations – Narrow substrate utilization range

 In

the ethanol fermenting bacterium Zymomonas mobilis hopanoids may have a role in adaptation of cell membranes to ethanol accumulation and to temperature changes which influence membrane functions.

Diploptene, a hopanoid compound

Cholesterol, a sterol compound.

Ethanol Fermentation in Zymomonas (ED Pathway)

Yeast using molasses as substrate Batch Process

2.0 g/l.h

Continuous Process

3.35 g/l.h

Immobilized cells under continuous conditions

28.6 g/l.h

The Immobilized process has already been placed in large-scale production in Japan