Microorganisms performing extracellular electron transfer (EET) show electroactivity and are the fundament of primary microbial electrochemical technologies (MET) (Schröder et al., 2015) as well as key players of geochemical cycles (Newman and Banfield, 2002; Melton et al., 2014). However, only a few electroactive microorganisms, like Geobacter or Shewanella, are studied in detail, e.g., for their electron transfer mechanisms (Gorby et al., 2006; Brutinel and Gralnick, 2012; Lovley, 2012). Many more species are only globally assigned to be electroactive (Koch and Harnisch, 2016), but mechanistic knowledge is generally missing and the natural importance of this trait not comprehensively understood.

However, there is no common definition of electroactivity and a genetic or metabolic marker or even a gold standard does not exist. This lack together with the high diversity of electroactive microorganisms—with regard to their phylogeny but also their physiology—challenges a systematic assessment and comparison (Koch and Harnisch, 2016). This difficulty is furthermore accelerated by the diversity of experimental setups and techniques exploited (Harnisch and Rabaey, 2012). The deficit of a stringent definition of electroactivity may sound purely academic from an application or engineering perspective. However, it is not. A consensus on electroactivity combined with good craftsmanship (Egli, 2015) for studying and engineering electroactive microorganisms as well as MET has to form the fundament of future research and development. The following treatise is certainly not comprehensive, but we will show that a better understanding of the linkage between EET, microbial metabolism, and system performance is necessary to form this fundament or in other words “To distil the essence of electroactivity.”