Clostridia: the importance of their exceptional substrate and metabolite diversity for biofuel and biorefinery applications.

Clostridia are anaerobic Firmicutes producing a large array of metabolites by utilizing simple and complex carbohydrates, such as cellulose, as well as CO2/H2 or CO. Their exceptional substrate diversity is enhanced by their ability to produce a broad spectrum of chemicals that can be used as precursors to or directly as biofuels and industrial chemicals. Genetic and genomic tools are under intense development, and recent efforts to metabolically engineer clostridia demonstrate their potential for biofuel and biorefinery applications. Pathway engineering to combine established substrate-utilization programs, such as for cellulose, CO2/H2 or CO, with desirable metabolic programs could lead to modular design of strains suitable for many applications. Engineering complex phenotypes--aerotolerance, abolished sporulation, and tolerance to toxic chemicals--could lead to superior bioprocessing strains.