Pretreatment and Enzymatic Hydrolysis of Lignocellulose

Native cellulolytic organisms, particularly the fungus Trichoderma reesei, are used industrially for the efficient production of cellulase mixtures [40]. 

Although early efforts toward achieving CBP have typically focused on ethanol, recent advances in microbial metabolic engineering have enabled the production of renewable chemicals and advanced bio-fuels that are compatible with existing engines and fuel distribution infrastructure [5]. This section focuses on the potential of microbial strains for use in CBP technologies. 

Clostridium thermoceUum demonstrated one of the fastest growth rates on crystalline cellulose by the use of a highly efficient cellulosome [54]. However, the construction of stable transgenic strains has been hmited by the lack of effective genetic tools. Tripathi etal. [55] developed positive and negative selectable markers to allow for the selection of strains that undergo allele replacement. The deletion of the Idh and pta genes, and a 2000-h adaptation, led to a 4.2-fold increase in the ethanol yield compared to wild-type cells. 

The capabilities of various fungi, including members of the genera Aspergillus, Rhizopus, Monilia, Neurospora, Fusarium, Tricoderma, and Mucor, have been explored for biomass utilization. T. reesei strains secrete large quantities of saccharolytic enzymes and thus may be candidate CBP microorganisms [56]. 

oryzae strains have several advantageous characteristics, including the ability to directly utilize non-pretreated cellulose and hemicellulose, tolerance to the inhibitors present in acid hydrolysates of lignocellulosic biomass, and the ability to produce fumaric acid, a four-carbon unsaturated dicarboxylic acid [57]. Recently, the white rot fungus Trametes hirsute was shown to be capable of directly fermenting starch, wheat bran, and rice straw to ethanol without prior acid or enzymatic hydrolysis [58]. 

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