A great number of bacteria are able to produce ethanol, although many of them generate multiple end products in addition to ethanol. Zymomonas mobilis is an unusual Gram-negative bacterium that has several appealing properties as a fermenting microorganism for ethanol production. It has a homoethanol fermentation pathway and tolerates up to 120 g/L ethanol. Its ethanol yield is comparable with S. cerevisiae, while it has much higher specific ethanol productivity (2.5 X) than the yeast. However, the tolerance of Z. mobilis to ethanol is lower than that of S. cerevisiae, since some strains of S. cerevisiae can produce ethanol to give concentrations as high as 18% of the fermentation broth. The tol­erance of Z. mobilis to inhibitors and low pH is also low. Similarly,

S. cerevisiae and Z. mobilis cannot utilize pentoses [14, 57]. Several genetic modifications have been performed for utilization of arabinose and xylose by Z. mobilis. However, S. cerevisiae has been more welcomed for industrial application, probably because of the industrial problems that may arise in working with bacteria. Separation of S. cerevisiae from fermentation media is much easier than separation of Z. mobilis, which is an important characteristic for reuse of the microorganisms in ethanol production processes.

Using genetically engineered bacteria for ethanol production is also applied in many studies. Ingram et al. [58] have reviewed metabolic engineering of bacteria for ethanol production. Recombinant Escherichia coli is a valuable bacterial resource for ethanol production. Construction of E. coli strains to selectively produce ethanol was one of the first suc­cessful applications of metabolic engineering. E. coli has several advan­tages as a biocatalyst for ethanol production, including the ability to ferment a wide spectrum of sugars, no requirements for complex growth factors, and prior industrial use (e. g., for production of recombinant protein). The major disadvantages associated with using E. coli cultures are a narrow and neutral pH growth range (6.0—8.0), less hardy cultures compared to yeast, and public perceptions regarding the danger of E. coli strains. Lack of data on the use of residual E. coli cell mass as an ingre­dient in animal feed is also an obstacle to its application [8].

Recently, the Japanese Research Institute of Innovative Technology for the Earth (RITE) developed a microorganism for ethanol production. The RITE strain is an engineered strain of Corynebacterium glutamicum that converts both pentose and hexose sugars into alcohol. The central metabolic pathway of C. glutamicum was engineered to produce ethanol. A recombinant strain that expressed the Z. mobilis gene coding for pyru­vate decarboxylase and alcohol dehydrogenase was constructed [59]. RITE and Honda jointly developed a technology for production of ethanol production from lignocellulosic materials using the strain. It is claimed that application of this strain by using engineering technology from Honda enables a significant increase in alcohol conversion efficiency, in comparison to conventional cellulosic—bioethanol production processes.