Page 501 - Refining Biomass Residues for Sustainable Energy and Bioproducts

458 Refining Biomass Residues for Sustainable Energy and Bioproducts

20.3.4.1 Escherichia coli

Escherichia coli has the natural ability to utilize a variety of sugars in both aerobic
and anaerobic conditions. In this regard, E. coli is used for producing biodiesel and
many other industrial products (Liu and Khosla, 2010). However, the development
of new molecular technology tools to identify the genetic variation and genetic
makeup of the organism helps in generating high amounts of biofuel (Ferrer et al.,
2009). Currently, ethanol is dominating in biofuel as an alternative source.
Additional advantage of using E. coli is its ability to metabolize both hexose and
pentose sugars (by glycolysis), the components of lignocellulosic material, whereas
many other microbes do not have the ability to use pentose sugar. Effective
implementation of genetic engineering can overcome this issue. In a study by
Zhang et al. (1995), the genes “pdc” and “adhB” from Zymomonas mobilis are
inserted in E. coli. These genes have been expressed in operon from plasmid which
is constitutively expressed in the production of ethanol using pET (expression
plasmid for T7 RNA polymerase) promoter. So, the genetically engineered E. coli
produces pyruvate decarboxylase and alcohol dehydrogenase II, which helps in pro-
ducing 95% ethanol without redox imbalance. Continuous generation of ethanol can
be achieved by constructing E. coli plasmid vector with “pods” and “adhB” gene
inserts with selective chloramphenicol resistance gene and integrated through
chromosome.

20.3.4.2 Zymomonas mobilis
Gram-negative, facultative anaerobe, rod-shaped bacterium belongs to class
Alphaproteobacteria that is a natural ethanologen which has many important
environmental characteristics to replace the natural fossil fuel (Yang et al., 2016). It
is regarded as the best alternative for fuel as it reduces the production cost of con-
trolling aeration during fermentation method (Yang et al., 2016). The nitrogen fixa-
tion ability of this organism does not affect the ethanol production; rather it
increases the productivity of ethanol. Z. mobilis has the ability to metabolize both
C 5 and C 6 sugars. The first recombinant strain of Z. mobilis was reported in 1995.
It was done by inserting “tal and tkt” genes from E. coli into Z. mobilis (Zhang
et al., 1995). Furthermore, genetic engineering in Z. mobilis has led the organism to
simultaneously utilize glucose, xylose, and arabinose for the production of ethanol
by fermentation method (Deanda et al., 1996). The “pdl and adh” genes are the
prominent genes that help in producing bioethanol and they have also been intro-
duced into other microorganism, such as E. coli, as reported earlier (Piriya et al.,
2012).

20.3.4.3 Bacillus subtilis
It is a Gram-positive rod-shaped bacterium that has added advantages of withstand-
ing low pH, high temperature, high salt concentrations, and other intolerant condi-
tions (Dien et al., 2003). Bacillus subtilis has the ability to produce butanediol,
lactate, acetate, and traces of ethanol in order to produce high amount of ethanol.