We recently discovered that wild-type E. coli strains are able to ferment glycerol in the absence of electron acceptors. We identified ethanol and 1,2-propanediol (1,2-PDO) as key fermentation products and proposed a new model for the fermentative utilization of glycerol in which: 1) production of 1,2-PDO provides a means to consume reducing equivalents generated in the synthesis of cell mass, thus facilitating redox balance, and 2) conversion of glycerol to ethanol, through a redox-balanced pathway, fulfills energy requirements by generating ATP via substrate-level phosphorylation. The knowledge base created by these studies has been instrumental in the design and implementation of metabolic engineering strategies to create microorganisms that efficiently convert glycerol to ethanol, hydrogen, formate, succinate, lactate, and 1,2-PDO, etc.
In the studies described above we used classical approaches to characterize the metabolic capabilities of glycerol-fermenting E. coli. Although very useful, these approaches have the shortcoming of restricting the discovery domain to those genes, proteins or pathways under investigation. To minimize these disadvantages, we are now using functional genomic (FG) tools such as DNA microarrays, 2-D Fluorescence Difference Gel Electrophoresis (2-D DIGE), and Metabolic Flux Analysis (MFA). In silico MFA, in combination with DNA microarrays and 2-D DIGE led us to discover additional metabolic and regulatory pathways involved in glycerol metabolism. Some of these pathways were of great value for the engineering of strains that efficiently produce fuels and chemicals from glycerol.