Glycerol is an abundant and inexpensive carbon source generated as inevitable by-product during the production of biodiesel. The higher reduced state of carbon in glycerol makes it an attractive target for the production of reduced chemicals such as ethanol, succinate, propanediols, etc. To fully realize this potential the use of anaerobic fermentation is required. Fermentative metabolism of glycerol, however, is restricted to very few microorganisms. E. coli, considered the platform of modern biotechnology, was thought for a longtime to metabolize glycerol only via respiration. Unlike previous reports, recent studies in our laboratory indicate that E. coli can anaerobically ferment glycerol (Biotechnol. Bioeng. 94 (5) 821, 2006). To further characterize this metabolic process we compared glycerol fermenting and non-fermenting cells using proteomics technology.

In this study, wild type E. coli MG1655 was grown at slightly acidic conditions, with only half point pH difference (pH 5.5 and 6) between the two cultures. This small change in pH made the cells to go from glycerol non-fermenting (pH 5.5) to glycerol fermenting (pH 6) state. To further identify the differences in metabolic pathways involved in the two conditions, 2D gel electrophoresis coupled with MALDI/TOF technology was used to identify the proteins, which were differentially expressed between the two conditions. Most of the differentially expressed proteins were from the central carbon metabolism of E. coli. Among these proteins, Tryptophanase and pyruvate kinase A were found to be expressed more than five times at the pH 6 condition as compared to pH 5.5. To further understand the metabolic pathways involved in the anaerobic fermentation of glycerol by E.coli, we are also studying some of the E.coli mutants using proteomics tools.