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Funding Information: This work was supported by national funds through FCT? Funda??o para a Ci?ncia e a Tecnologia, I.P. Project UIDB/04612/2020, UIDP/04612/2020 and PTDC/BIA-BQM/30176/2017, and by the European Union's Horizon 2020 research and innovation programme under grant agreement No 810856. Funding Information: This work was supported by national funds through FCT– Fundação para a Ciência e a Tecnologia, I.P., Project UIDB/04612/2020, UIDP/04612/2020 and PTDC/BIA-BQM/30176/2017, and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 810856. Publisher Copyright: © 2020 The Author(s) Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

The growing interest on sustainable biotechnological processes for the production of energy and industrial relevant organic compounds have increased the discovery of electroactive organisms (i.e. organisms that are able to exchange electrons with an electrode) and the characterization of their extracellular electron transfer mechanisms. While most of the knowledge on extracellular electron transfer processes came from studies on Gram-negative bacteria, less is known about the processes performed by Gram-positive bacteria. In contrast to Gram-negative bacteria, Gram-positive bacteria lack an outer-membrane and contain a thick cell wall, which were thought to prevent extracellular electron transfer. However, in the last decade, an increased number of Gram-positive bacteria have been found to perform extracellular electron transfer, and exchange electrons with an electrode. In this mini-review the current knowledge on the extracellular electron transfer processes performed by Gram-positive bacteria is introduced, emphasising their electroactive role in bioelectrochemical systems. Also, the existent information of the molecular processes by which these bacteria exchange electrons with an electrode is highlighted. This understanding is fundamental to advance the implementation of these organisms in sustainable biotechnological processes, either through modification of the systems or through genetic engineering, where the organisms can be optimized to become better catalysts.