1 Scientific REPORTS | 7:44334 | DOI: 10.1038/srep44334 www.nature.com/scientificreports Cooperative growth of Geobacter sulfurreducens and Clostridium pasteurianum with subsequent metabolic shift in glycerol fermentation Roman Moscoviz, Florence de Fouchécour, Gaëlle Santa-Catalina, Nicolas Bernet & Eric Trably Interspecies electron transfer is a common way to couple metabolic energy balances between diferent species in mixed culture consortia. Direct interspecies electron transfer (DIET) mechanism has been recently characterised with Geobacter species which couple the electron balance with other species through physical contacts. Using this mechanism could be an efcient and cost-efective way to directly control redox balances in co-culture fermentation. The present study deals with a co-culture of Geobacter sulfurreducens and Clostridium pasteurianum during glycerol fermentation. As a result, it was shown that Geobacter sulfurreducens was able to grow using Clostridium pasteurianum as sole electron acceptor. C. pasteurianum metabolic pattern was signifcantly altered towards improved 1,3-propanediol and butyrate production (+37% and +38% resp.) at the expense of butanol and ethanol production (-16% and -20% resp.). This metabolic shift was clearly induced by a small electron uptake that represented less than 0.6% of the electrons consumed by C. pasteurianum. A non- linear relationship was found between G. sulfurreducens growth (i.e the electrons transferred between the two species) and the changes in C. pasteurianum metabolite distribution. This study opens up new possibilities for controlling and increasing specifcity in mixed culture fermentation. To sustain their growth and maintenance, microorganisms perform oxidative and reductive reactions inside their cells. Tese redox reactions consist in electron fows coming from an electron donor that are stepwise transferred to a terminal electron acceptor (e.g. O 2 in aerobic respiration) with an overall release of free energy. However, one single species is not always able to perform the entire cascade of reactions. In this case, some microorganisms couple their electron fows with other species via interspecies electron transfer (IET) to carry out reactions that would otherwise be thermodynamically unfavourable 1,2 . A well described example is the IET existing between archaea and bacteria during methanogenesis through the difusion of H 2 or formate 2,3 . More recently, direct interspecies electron transfer (DIET) that does not proceed through the difusion of electron carriers has been discovered. During DIET, electrons are transferred via biological electrical connections between electron-donor (exoelectrogens) and electron-acceptor (electrotrophs) microorganisms 2,4,5 . Contacts between the two partners can be ensured by the establishment of a bioflm on a conductive material 1,5 (e.g. iron oxides or carbon materials) or by connecting species with pili with metallic-like conductive properties 1,2,4 . Tese pili, named nanowires, can be produced by iron-reducing bacteria such as Geobacter metallireducens 6 or Geobacter sulfurreducens 7 that are even able to connect bacteria up to a centimetre scale 8 . Instead of using Fe 3+ , these species were reported to be able to transfer their electrons to other species, such as denitrifying bacteria 9,10 or methanogens 11 . Interestingly, one of the two partners of a DIET can be replaced by an electrode that act as an artifcial electron donor 12,13 (cathode) or acceptor 14 (anode). Tis is the basis of bio-electrochemical systems (BESs), processes that have been designed to take advantage of electro-active bacteria to produce electricity, chemicals or other services 15 . For instance, exoelectrogens (e.g. Geobacter species) can generate electrical power in microbial fuel cells while oxidizing organic matter from waste 14 . Methanogens or denitrifying bacteria (electrotrophs) are also LBE, INRA, 102 Avenue des étangs, 11100 Narbonne, France. Correspondence and requests for materials should be addressed to E.T. (email: eric.trably@inra.fr) received: 05 December 2016 accepted: 26 January 2017 Published: 13 March 2017 OPEN