Glycerol can be converted into higher value products by either biological or chemical trans­formations. Biological conversions are generally preferred to chemical conversions because biological conversions have higher reaction specificities, lower reaction temperatures and pressure, and fewer chemical contaminants than chemical conversions. Crude glycerol gener­ated via biotransformations can be used as a carbon source for other microbial fermentations. The carbon atoms in glycerol molecules are highly reduced, and the conversion of glycerol to glycolytic intermediates generates twice the amount of reducing equivalents generated by glucose or xylose metabolism. For example, conversion of 1 mol of glycerol (a three-carbon molecule) to phosphoenol pyruvate (PEP) or pyruvate generates 2 mol of NADH, while conversion of 0.5 mol of glucose (a six-carbon molecule) or 0.6 mol of xylose (a five-carbon molecule) to PEP or pyruvate generates only 1 mol of NADH. As a result, yields of fuels and chemicals are higher when synthesized from glycerol than monosaccharides.

Uptake of glycerol across the cytoplasmic membrane by cells can occur via facilitated diffusion and active transport. In E. coli, the uptake of glycerol is through facilitated diffu­sion, mediated by an integral membrane protein, the glycerol facilitator GlpF (Heller et al. 1980; Voegele et al. 1993) . This intracellular glycerol is phosphorylated by the glycerol kinase and the glycerol-3-phosphate formed remains trapped inside the cell. In S. cerevisiae, the uptake of glycerol is mediated by either facilitated diffusion or active transport (Wang et al. 2001).

Although glycerol is commonly used by many microorganisms under aerobic condition, the highly reduced nature of glycerol can be exploited for the production of numerous bio­products. Few members of the Enterobacteriaceae family such as Citrobacter freundii (Homann et al. 1990) and Klebsiella pneumoniae (Forage and Foster 1982; Biebl et al. 1998) have been reported to ferment glycerol in a 1,3-propanediol (1,3-PDO)-dependent manner. Species such as Clostridium pasteurianum (Luers et al. 1997) Biebl 2001), Clostridium butyricum (Biebl 1991), Enterobacter agglomerans (Barbirato et al. 1996), Enterobacter aerogenes (Ito et al. 2005), and Lactobacillus reuteri (Talarico et al. 1990) have also been reported to ferment glycerol. A brief summary of the fermentation products of crude glycerol is given in Figure