There is an ever-growing demand for new and improved bioethanol production microorganism strains. Desirable characteristics of bioethanol production microorganisms are listed in Table 1.

Ethanol production microorganisms, mainly Zymomonas mobilis and Saccharomyces cerevisiae, are potential candidates for bioethanol productions because they showed many of the characteristics presented in the table 1. However, Zymomonas mobilis strains have attracted much attention because their growth rate is higher than that of Saccharomyces cerevisiae,

conventionally used microorganisms for commercial bioethanol production. Zymomonas mobilis has been used in tropical areas for making alcoholic beverages from plant sap [2], but its narrow spectrum of fermentable carbohydrates has hampered its industrial exploitation [3]. Several researchers have taken on the challenger on developing recombinant organisms, including: S. cerevisiae, Z. mobilis, Escherichia coli, Klebsiella oxytoca and Erwinia herbicola [4-5], but the bioethanol production from biomass materials by genetically engineered strains has not yet reached a sufficient level for commercial application [6]. Zymomonas cells are gram­negative rods; a minority of the strains are motile, with 1 to 4 polar flagella. These organisms need glucose, fructose, or (for some strains) sucrose in the growth medium. They are very unusual microorganisms since they ferment these sugars anaerobically by way of the Entner-Doudoroff mechanism, followed by pyruvate decarboxylation. The oxidation — reduction balance between G6P dehydrogenase and triosephosphate dehydrogenase on one hand and ethanol dehydrogenase on the other, is mediated through NAD+. Sugar fermentation is accompanied by formation of a small amount of lactic acid, with traces of acetaldehyde and acetoin [2].

The simplified fermentation process is:

C6H12O6 + (carbon source) ^ 1.8 CH3CH2OH + 1.8 CO2 +

+ 0.2CH3CH (OH)COOH + 0.22 CH2O + ATP + 32.7 kcal (1)

The molar growth yield indicates that Zymomonas is only about 50% efficient in converting its carbon and energy sources. Growth is partially uncoupled. About 2% of the glucose substrate is the source of about half of the cellular carbon. Several amino acids also serve as carbon sources. Some strains grow only anaerobically; others display various degrees of microaerophily. Apparently, the main effect of oxygen is the oxidation of part of the ethanol which converts into acetic acid. Most strains are alcohol tolerant (10%) and grow in up to 40% glucose. The wide pH for growth range from 3.5 to 7.5, and acid tolerance are quite typical. This bacterium has been isolated from fermenting agave sap in Mexico, from fermenting palm saps in Zaire, Nigeria, and Indonesia, from fermenting sugarcane juice in Northeastern Brazil. Undoubtedly, they are important contributors to the fermentation of plant saps in many tropical areas of the America, the Africa, and Asia.

cerevisiae cells measure 3-7 microns wide and 5-12 microns long. It has elliptic, round and oval shapes and reproduces is by a division process known as budding [7]. It is believed that S. cerevisiae was originally isolated from the skin of grapes [8]. Its optimum temperature growth range is 30° C [9]. S. cerevisiae is tolerant of a wide pH range (2.4-8.2), being the optimum pH for growth between values of 3.5 to 3.8 [10]. In addition, S. cerevisiae is high growth rate (0.5 h-1) in the yeast group. With respect to S. cerevisiae nutritional requirements, all strains can grow aerobically on glucose, fructose, sucrose, and maltose and fail to grow on lactose and cellobiose. Also, all strains of S. cerevisiae can use ammonia and urea as the sole nitrogen source, but cannot use nitrate since they lack the ability to reduce them to ammonium ions. They can also use most amino acids, small peptides and nitrogen bases as a nitrogen sources [11]. S. cerevisiae have a phosphorus requirement, assimilated as a dihydrogen phosphate ion, and sulfur, which can be assimilated as a sulfate ion or as organic sulfur compounds, such as the amino acids: methionine and cysteine. Some metals, such as magnesium, iron, calcium and zinc are also required for good growth of this yeast.

Alcoholic fermentation by yeast consists of three main stages: (1) transporting sugars within the cell, (2) transforming sugars into pyruvate through glycolysis pathway and finally (3) converting acetaldehyde to ethanol.

The simplified fermentation process is:

C6H12O6 + (carbon source) ^ 2CH3CH2OH + 2 CO2 + 2 ATP + 25.5 kcal (2)