A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by using bacteria and mimicking bacterial interactions found in nature. MFCs can be grouped into two general categories, those that use a mediator and those that are mediator-less. The first MFCs, demonstrated in the early 20th century, used a mediator: a chemical that transfers electrons from the bacteria in the cell to the anode. Mediator-less MFCs are a more recent development dating to the 1970s; in this type of MFC the bacteria typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode. Since the turn of the 21st century MFCs have started to find a commercial use in the treatment of wastewater.