Fermentation Using Bacteria
Bacteria such as Zymomonas mobilis, Escherichia coli and Klebsiella oxytoca have attracted particular interest in ethanol fermentation given their rapid fermentation which can be minutes compared to hours or days for yeasts [43]. Zymomonas mobilis is a Gram-negative bacterium belonging to the genus Zymomonas and is notable for its bioethanol production capabilities, which surpass yeast in some aspects. Z. mobilis is well recognized for its ability to efficiently produce ethanol at high rates from glucose, fructose, and sucrose. It was originally isolated from alcoholic beverages like African palm wine and Mexican pulque, and was also a contaminant of cider and beer in European countries.
Zymomonas mobilis degrades sugars to pyruvate using the Entner — Doudoroff pathway. The Entner-Doudoroff pathway describes an alternate series of reactions that catabolize glucose to pyruvate using a set of enzymes different from those used in either glycolysis or the pentose phosphate pathway [44]. The distinct features of this pathway are that it has a net yield of one ATP for every glucose molecule processed, as well as one NADH and one NADPH. By comparison, glycolysis has a net yield of two ATP and two NADH for every one glucose molecule processed [44]. The pyruvate is then fermented to produce ethanol and carbon dioxide as the only products, similar to yeast.
The advantages of Zymomonas mobilis over Saccharomyces cerevi — siae with respect to producing ethanol from lignocellulosic biomass can be summarized as follows [45]:
1. Higher sugar uptake and higher ethanol yield
2. Higher ethanol tolerance up to 16% (v/v)
3. Does not require controlled addition of oxygen during the fermentation
Comparative performance trials on glucose have shown that Zymomonas mobilis can achieve 5% higher bioethanol yields and up to five-fold higher bioethanol volumetric productivity compared to wild type Saccharomyces cerevisiae yeast [46,45]. Zymomonas mobilis can efficiently produce bioethanol from the hexose sugars glucose and fructose. However, wild-type Z. mobilis cannot ferment C-5 sugars like xylose and arabinose, which are important components of lignocellulosic hydrolyzates. Another disadvantage is that, unlike E. coli and yeast, Zymomonas mobilis cannot tolerate toxic inhibitors present in lignocellulosic hydrolyzates such as acetic acid and various phenolic compounds. Concentration of acetic acid in lignocel — lulosic hydrolyzates can be as high as 1.5% (w/v), which is well above the tolerance threshold of Zymomonas mobilis. Therefore efficient detoxification steps are essential in using this type of bacteria in the fermentation step.