E. coli use fermentation pathways as a final option for energy metabolism, as they produce very little energy in comparison to respiration. 
Mixed acid fermentation in E. coli occurs in two stages. These stages are outlined by the biological database for E. coli, EcoCyc.
The first of these two stages is a glycolysis reaction. Under anaerobic conditions, a glycolysis reaction takes place where glucose is converted into pyruvate:
      glucose → 2 pyruvate
There is a net production of 2 ATP and 2 NADH molecules per molecule of glucose converted. ATP is generated by substrate-level phosphorylation. NADH is formed from the reduction of NAD.
In the second stage, pyruvate produced by glycolysis is converted to one or more end products via the following reactions. In each case, both of the NADH molecules generated by glycolysis are reoxidized to NAD. Each alternative pathway requires a different key enzyme in E. coli. After the variable amounts of different end products are formed by these pathways, they are secreted from the cell.