The current fermentative production of butanol is not cost effective because of (1) a spore-forming life cycle, (2) butanol toxicity, (3) slow growth and instability of the producing strains and (4) production of other unwanted byproducts including butyrate, acetate, acetone, and ethanol [31]. In addition, no commercial microbes are available to ferment various lignocellulosic hydrolyzate mixtures into butanol. Thus, new microbes are needed for fermentative conversion of these hydrolyzates to butanol biofuel.

In the recent years, the fermentative production of butanol has been demonstrated in engineered strains of Escherichia coli [4, 30, 43] and Saccharomyces cerevisiae [71]. The entire butanol production pathway from Clostridium has been recon­structed and introduced into these model hosts. More recently, the pathway recon­struction strategy was applied to more robust and butanol tolerant species including Pseudomonasputida, Bacillus subtilis [43], Lactobacillus brevis [7], Lactobacillus buchneri [35], and Corynebacterium glutamicum [69]. Although the polycistronic expression of the butanol production pathway genes are achieved in these robust host cells, the butanol titers of these recombinant organisms are relatively low (Table 1) and has yet to exceed 19.50 g/L, a production level that can be achieved by Clostridium species [57] .

Three of the highest butanol producing strains are the engineered E. coli strains JCL187, EB4.F, and BUT2, which can produce 552, 580, and 1,200 mg/L of butanol, respectively (Table 1). Although the E. coli BUT2 was reported as producing more butanol, the cells were first grown aerobically and later resuspended for anaerobic fermentation. Furthermore, these strains suffer from butanol toxicity (very sensitive to butanol) and can be killed by the accumulation of no more than 15 g/L butanol [32]. So far, no breakthrough improvement ofbutanol production strains from ligno — cellulosic biomass hydrolyzates has been reported and yet, more research is needed for strain development.