http://synbioproject.tech/cpi/applications/microbial-fuel-cells/
A microbial fuel cell (MFC) converts chemical energy, available in a bio-convertible substrate, directly into electricity. To achieve this, bacteria are used as a catalyst to convert substrate into electrons.
Bacteria are very small (size appr. 1 µm) organisms which can convert a huge variety of organic compounds into CO2, water and energy. The micro-organisms use the produced energy to grow and to maintain their metabolism. However, by using an MFC, we can harvest a part of this microbial energy in the form of electricity.