Classification paragraph by Oxana Koshova

Microbial fuel cells: principles of organization and prospects of use

Microbial fuel cells (MFCs) can be divided into mediator and mediator-less microbial fuel cells. The first class of MFCs is electrochemically inactive. The electron transfer from microbial cells of this type to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, and neutral red. According to the mediator, different types of MFCs are distinguished, for example, thionine or methyl viologen MFCs. Mediator-less microbial fuel cells, on the other hand, does not require a mediator but use electrochemically active bacteria to transfer electrons to the electrode. Among the electrochemically active bacteria are Geothrix fermentans, Desulfobulbus propionicus, Rhodoferax ferrireducens, Shewanella putrefaciens, and Aeromonas hydrophila. Similarly, there are Geothrix-based MFCs or Desulfobulbus-based MFCs. While most of the mediators available are expensive and toxic, mediator-less microbial fuel cells are more preferable.

Reference:
http://en.wikipedia.org/wiki/Microbial_fuel_cell