The quest for cellulolytic organisms has recently gained increased interest because of the potential to circumvent the cost of enzymes used for cellulose hydrolysis. An ideal host for cellulosic ethanol production should possess certain traits, such as a broad substrate range (utilizing both pentoses and hexoses), high productivity, and tolerance to both ethanol and toxic compounds of lignin (Fischer et al. 2008). In order to identify desirable organisms for cellulosic ethanol production, naturally evolved cellulose-degrading microbes have been characterized from several sources, including the rumen of cattle and the gut of insects, and even from marine environments (Hess et al. 2011). However, most of these microbes cannot be cultivated with synthetic media in the laboratory. Hence, DNA isolates were directly sequenced and putative carbohydrate-hydrolyzing genes were identified (Hess et al. 2011). With this metagenomic approach, identification of microbes suitable for cellulosic fuel production has not been possible, because our current knowledge of the genes is limited.

Well-characterized native cellulolytic organisms include Cellulomonas fimi, Fibrobacter succinogenes, Ruminococcus albus, and C. thermocellum. Among these, C. thermocellum is of considerable importance, because it is recognized as a "cellulose-using specialist" (Zhang and Lynd 2005). Cellulolytic organisms produce many isoforms of the three different cellulases. T. reesei, for example, can secrete five endoglucanases, two cellobiohydrolases, and two ^-glucosidases. Apart from cellulases, these organisms also secrete adhesion proteins like glycocalyx, which enables strong adhesion of the cellulolytic organisms to cellulose (Lynd et al. 2002).

Despite the diversity of cellulolytic organisms, none of these organisms are known to produce ethanol efficiently (Xu et al. 2009). Even as the search for a cellulolytic organism with the ability to produce ethanol continues, another strategy would be to engineer efficient ethanol production into cellulolytic organisms such as Clostridium spp. (Lynd et al. 2005). However, a lack of proper genetic tools for manipulating these uncommon laboratory strains and very limited knowledge of their genotypes have resulted in a need to engineer the cellulolytic ability into efficient ethanol producers such as S. cerevisiae, E. coli and Z. mobilis.

Consolidated Bioprocessing
Glucose
Cellulose Hydrolysis
о Oo°e
Ethanol
— Crude Cellulases — Cellulose — Ethanologenic Microbes