were the same for the pure-culture and the co-culture. The fermen-
tation reaction of L. lactis occurred in planktonic state and did not
inﬂuence the bioelectrochemical reaction of S. oneidensis, besides
the provision of lactate as the electron donor (i.e., a neutral rela-
tionship). Thus, the interaction between the two organisms oc-
curred only at the substrate level. Our work supports the
hypothesis that fermenting bacteria only provide intermediate
products to ARB, such as from the genera Shewanella and Geobacter,
in BESs that are dominated by direct electron transfer. Under such
conditions when external redox shuttles are absent, the very small
electric currents generated by fermenters are inconsequential. This
ﬁnding is in agreement with the results of Ren et al. (2007) for a co-
culture of G. sulfurreducens and C. cellulolyticum, but not with the
study of Read et al. (2010), who found a negative impact of the fer-
menter C. acetobutylicum and a positive, synergistic effect of the
fermenter E. faecium on the current production by S. oneidensis,
G. sulfurreducens, and P. aeruginosa. It is, thus, clear that the inter-
actions between ARB and fermenters cannot be simply predicted,
especially since some ARB only perform direct electron transfer,
some only interact with the electrode through redox mediators,
and some are capable of both. This opens many possible ways of
interaction between the ARB and fermenters. While substrate level
interactions between microorganisms in a mixed-culture BES are
important to uncover (such as in our study), the identiﬁcation
and investigation of true synergistic fermenter–ARB relationships
will be much more essential for the improvement of BES
The understanding of metabolic network relationships in
mixed-culture BES, which can be of neutral, positively enhancing,
or negatively impeding kind, becomes an important step in the
advancement of BES development. Here, we studied the relation-
ship between the homolactic fermenter L. lactis, which converts
glucose to lactate, and the electricigen S. oneidensis, which converts
lactate into electric current at conditions that are pertinent to con-
ventional BES operation. With electrochemical, metabolic, and
gene expression analysis we determined that the two cultures
establish a pure food-based relationship, because the physiology
and electrochemical activity of S. oneidensis was similar regardless
of the presence of L. lactis.
Financial support for this work was provided through a speciﬁc
collaborative agreement between LTA and the Bioenergy Research
Unit, USDA, Agricultural Research Service, Peoria, IL and the Na-
tional Science Foundation through grant no. 0939882. D.S. and
Q.K.B. are partially supported by US Department of Energy grants
DE-FG02-07ER64388 and DE-FG02-07ER64483. We thank Pat
O’Bryan and Bruce Dien of the USDA-ARS, Peoria, IL, for their help
with the HPLC analysis, Mike Veith of Washington University in
St. Louis, St. Louis, MO, for his help with SEM imaging, and Gretta
Serres of the Marine Biological Laboratory, Woods Hole, MA for the
provision of the S. oneidensis gene annotation.
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