Engineering Clostridium for improved solvent production: recent progress and perspective

Biologically mediated processes seem promising for energy conversion, in particular for the conversion of lignocellulosic biomass into fuels. Although processes featuring a step dedicated to the production of cellulase enzymes have been the focus of most research efforts to date, consolidated bioprocessing (CBP)--featuring cellulase production, cellulose hydrolysis and fermentation in one step--is an alternative approach with outstanding potential. Progress in developing CBP-enabling microorganisms is being made through two strategies: engineering naturally occurring cellulolytic microorganisms to improve product-related properties, such as yield and titer, and engineering non-cellulolytic organisms that exhibit high product yields and titers to express a heterologous cellulase system enabling cellulose utilization. Recent studies of the fundamental principles of microbial cellulose utilization support the feasibility of CBP.

Consolidated bioprocessing, or CBP, the conversion of lignocellulose into desired products in one step without added enzymes, has been a subject of increased research effort in recent years. In this review, the economic motivation for CBP is addressed, advances and remaining obstacles for CBP organism development are reviewed, and we comment briefly on fundamental aspects. For CBP organism development beginning with microbes that have native ability to utilize insoluble components of cellulosic biomass, key recent advances include the development of genetic systems for several cellulolytic bacteria, engineering a thermophilic bacterium to produce ethanol at commercially attractive yields and titers, and engineering a cellulolytic microbe to produce butanol. For CBP organism development, beginning with microbes that do not have this ability and thus requiring heterologous expression of a saccharolytic enzyme system, high-yield conversion of model cellulosic substrates and heterologous expression of CBH1 and CBH2 in yeast at levels believed to be sufficient for an industrial process have recently been demonstrated. For both strategies, increased emphasis on realizing high performance under industrial conditions is needed. Continued exploration of the underlying fundamentals of microbial cellulose utilization is likely to be useful in order to guide the choice and development of CBP systems. Copyright © 2011 Elsevier Ltd. All rights reserved.

Clostridium ljungdahlii is an anaerobic homoacetogen, able to ferment sugars, other organic compounds, or CO(2)/H(2) and synthesis gas (CO/H(2)). The latter feature makes it an interesting microbe for the biotech industry, as important bulk chemicals and proteins can be produced at the expense of CO(2), thus combining industrial needs with sustained reduction of CO and CO(2) in the atmosphere. Sequencing the complete genome of C. ljungdahlii revealed that it comprises 4,630,065 bp and is one of the largest clostridial genomes known to date. Experimental data and in silico comparisons revealed a third mode of anaerobic homoacetogenic metabolism. Unlike other organisms such as Moorella thermoacetica or Acetobacterium woodii, neither cytochromes nor sodium ions are involved in energy generation. Instead, an Rnf system is present, by which proton translocation can be performed. An electroporation procedure has been developed to transform the organism with plasmids bearing heterologous genes for butanol production. Successful expression of these genes could be demonstrated, leading to formation of the biofuel. Thus, C. ljungdahlii can be used as a unique microbial production platform based on synthesis gas and carbon dioxide/hydrogen mixtures.