## 2 Fermentation Pathways and Hosts for Ethanol Production  
Ethanol is produced from glucose via fermentative consumption of pyruvate [2]. Glycolysis is a metabolic process that converts glucose to partially oxidized product, pyruvate, while supplying ATP for biomass production. Subsequently, under anaerobic conditions, pyruvate can be fermented to ethanol by sequential reactions of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) while losing one carbon as carbon dioxide (CO\({}_{2}\)). The ethanol fermentation process has been extensively studied and exploited in _Saccharomyces cerevisiae_ (yeast) and _Escherichia coli_[1, 3], due to the relative technological maturity in genetically engineering these microbes. Other species have also been considered as production hosts due to advantages of their native enzymes and pathways. For instance, _Zymommonas mobilis_ has been suggested as an alternative host to yeast due to its advantage for ethanol yield since it utilizes the Entner-Doudoroff (ED) pathway instead of Embden-Meyerhof-Parnas (EMP) pathway for glycolysis [4] (Figure 2). Although the EMP pathway is a major glycolysis route in most eukaryotes and prokaryotes, glycolysis pathways are much more diverse in prokaryotes [5]. Among variants of the glycolysis pathway, the ED pathway is the most abundant route together with EMP pathway in some prokaryotes such as _Z. mobilis_[6]. While the EMP pathway produces two ATPs from each glucose molecule consumed, the ED pathway produces only one ATP molecule from one glucose molecule. Given that ATP is tightly coupled with anabolism and cell growth, ED pathway-utilizing _Z. mobilis_ produces less biomass than EMP pathway-dependent species such as _S. cerevisiae_ and _E. coli_. Consequently, _Z. mobilis_ has more available carbons for ethanol fermentation with 2.5-fold higher specific ethanol productivity than that of _S. cerevisiae_, and produces up to 97% theoretical yield [7]. In addition, _Z. mobilis_ has been engineered to co-utilize glucose, mannose and xylose, expanding their capability for ethanol fermentation of LCB-derived sugars [8]. Clostridia, on the other hand, have advantages over _S. cerevisiae_ because Clostridia naturally secrete enzymes that are capable of hydrolyzing complex carbohydrates (oligosaccharides and polysaccharides) into fermentable sugars and utilizing both hexose and pentose [9, 10]. As a result, Clostridia have been suggested as a candidate host for biofuel production from LCB for consolidated bioprocessing (CBP). However, Clostridia are strict anaerobes and their growth is relatively slower than other microbial hosts, which makes fermenter operation difficult. Various aspects of Clostridia for industrial use have been well summarized in a previous review [11].  
In this section, we briefly introduce various aspects on selection of microbial hosts for ethanol production: energetics of the glycolysis pathways, available genetic engineering tools, flexibility in sugar utilization and compatibility to effective fermenter operation. The following section will discuss metabolic engineering strategies that have been applied to optimize and improve microbial hosts for ethanol production.