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1
Supporting information
Periodic polarization of electroactive biofilms
increases current density and charge carriers
concentration while modifying biofilm structure
Xu Zhanga, Antonin Prévoteaua,*, Ricardo O. Lourob, Catarina M. Paqueteb, Korneel
Rabaeya,*
a. Center for Microbial Ecology and Technology (CMET), Ghent University, Coupure Links
653, 9000, Ghent, Belgium.
b. Universidade Nova de Lisboa, Av da Republica (EAN), 2780-157 Oeiras, Portugal.
Corresponding Authors
*Phone: 32 (0)9 264 59 76 ; e-mail: Korneel.Rabaey@UGent.be.
*Phone: 32 (0)9 264 59 73 ; e-mail: Antonin.Prevoteau@UGent.be.
Pages:25
Figures: 20
Table: 2
2
Table of Contents
Fig. S1. Scheme of the 8-electrode reactor and uncompensated resistance (Ru) recorded
for the 8-electrode (n = 5 measurements per electrode). .................................................... 3
Fig. S2. Quasi-saturation in acetate. Impact of rotating speed and acetate concentration
on the steady state current of a continuously polarized EAB. ............................................ 4
Fig. S3. Limited impact of capacitive current in the total amount of charge recorded per
half-period of polarization (Qcycle). ..................................................................................... 5
Fig. S4. Chronoamperograms recorded during periodic polarization for the triplicates
EAB-10s, after 10 days of growth. ..................................................................................... 6
Table S5. Qcycle and Γ for the EABs under periodic polarization. ...................................... 7
Fig. S6. Growth of mature EABs for the preliminary experiment. .................................... 8
Fig. S7. Electrochemical responses of mature EABs with different tc. .............................. 9
Fig. S8. Evolution of OCPf and estimated fraction of redox cofactors reduced at the
electrode interface (CR/CT) with different tc. .................................................................... 10
Fig. S9. Cyclic voltammograms of mature EABs for preliminary experiment. ............... 11
Fig. S10. Schematized, conjectured concentration profiles for the reduced form of the
redox charge carrier .......................................................................................................... 12
Fig. S11. Representative transient current at the start of discharge for the different
periodic polarizations. ....................................................................................................... 13
Fig. S12. Recording the apparent steady-state catalytic current for EABs grown under
periodic polarization. ........................................................................................................ 14
Fig. S13. Charge surface coverage estimation (Γ, in mole- cm−2)..................................... 15
Fig. S14. Representative non-turnover CVs of EABs grown under continuous (control)
and periodic polarization. .................................................................................................. 16
Fig. S15. Proportional relationship between jss and Γ (top); and between jss and C
(bottom). ............................................................................................................................ 17
Fig. S16. Representative Cottrell plots for the discharge of the different EABs. ............. 19
Fig. S17. SDS-PAGE gel (15 % acrylamide) with denatured DNA samples for each EAB.
........................................................................................................................................... 20
Fig. S18. Ratio of heme over total protein content (top) and optical absorption spectra of
heme from the different EABs (bottom). .......................................................................... 21
Fig. S19. Calibration curves of total protein. .................................................................... 22
Table S20. Morphologic characteristics of the EABs. ..................................................... 23
Fig. S21. Microbial community composition of EABs at the end of the 10 days operation.
........................................................................................................................................... 24
Fig. S22. Fraction of Geobacter determined by Illumina sequencing of different EABs. 26
3
Fig. S1. Scheme of the 8-electrode reactor and uncompensated resistance (Ru) recorded for
the 8-electrode (n = 5 measurements per electrode).
Considering the non-central position of the reference electrode in the reactor, the
uncompensated resistance between each working electrode and the reference electrode was
estimated by applying a current interruption method (Bard and Faulkner 2001). Ru was similar
for each electrode, between 13 and 20 Ω. Considering the maximum current generation of
EABs (Imax ~ 2 mA), the maximum difference in ohmic drop (= Imax × Ru) was small (7 Ω × 2
mA = 14 mV) and could not substantially impact the electrochemical measurements since the
potential of polarization (− 0.1 V) is well above the potential at which the anodic plateau is
reached (around − 0.2 V, see Fig. S3). The high reproducibility of electrochemical recordings
between randomly distributed triplicates further confirms that the electrode position did not
impact the measurements.
Reactor 1
Ru / Ω
Electrode
Ru / Ω
E1
13.1 ± 0.9
E5
20.0 ± 0.4
E2
15.8 ± 0.8
E6
17.4 ± 0.8
E3
19.3 ± 0.8
E7
17.3 ± 1.1
E4
18.8 ± 0.9
E8
13.8 ± 0.7
Reactor 2
Ru / Ω
Electrode
Ru / Ω
E1
13.9 ± 0.9
E5
20.2 ± 1.3
E2
17.0 ± 1.0
E6
19.8 ± 1.1
E3
18.5 ± 1.1
E7
17.5 ± 0.6
E4
19.3 ± 0.9
E8
14.2 ± 1.3
4
1
2
Fig. S2. Quasi-saturation in acetate. Impact of rotating speed and acetate concentration on the
3
steady state current of a continuously polarized EAB.
4
The steady state current increased about 2.6 % when the rotation speed of the magnetic stirrer
5
was increased from 100 to 500 rpm (top). The current declined by 3.2 % after adding 7 mM
6
acetate, from 23 to 30 mM (bottom). These small relative impacts of convection and acetate
7
addition (even negative in the latter case) illustrate the quasi-saturation of acetate for the
8
reaction rate.
9
5
10
11
12
Fig. S3. Limited impact of capacitive current in the total amount of charge recorded per half-
13
period of polarization (Qcycle).
14
The applied polarization potential of an EAB-10s was decreased from + 0.3 V to – 0.1 V with
15
a negative increment of 0.1 V every 3 periods (panel (a), the red lines show the polarization
16
half-periods). The polarization potential did not impact the value of the OCPf which stays
17
constant at – 0.50 V (dotted black line), suggesting that the applied potential did not impact
18
significantly the charging of the EAB during the OCP half-period. This was expected since all
19
the applied potentials are located on the plateau current corresponding to the electron mass-
20
transfer limitation (see panel (c)).
21
The corresponding amounts of charges Qcycle recorded for 10 s of polarization at the different
22
potentials are plotted on panel (b). Qcycle increased quasi-linearly with the polarization
23
potential because of the increasing capacitive current, itself dependent of the amplitude of the
24
potential step between OCPf (− 0.5 V) and the applied potential. However, the variation of
25
Qcycle with the applied potential is particularly small, increasing only by 0.46 %, when the
26
applied potential increased from − 0.1 V to + 0.3 V. This proves that the extreme majority of
27
the charges collected by the electrode is faradaic and originates from the metabolism of the
28
EAB.
29
6
30
Fig. S4. Chronoamperograms recorded during periodic polarization for the triplicates EAB-
31
10s, after 10 days of growth.
32
The CAs shows 3 successive periods. The CAs for the 3 EABs are superimposed, showing a
33
high level of reproducibility within triplicates. This reproducibility is representative of the
34
data collected for the different groups of EABs.
35
0
1
2
3
020 40 60
j/ mA cm−2
t/ s
E4
E5
E6
7
EAB-1s
EAB-10s
EAB-60s
EAB-300s
Qcycle [mC.cm−2]
0.90 ± 0.03
12.33 ± 0.12
4.70 ± 0.25
22.02 ± 0.64
Γ [10−8 mole-.cm−2]
3.08 ± 0.11
3.73 ± 0.17
0.26 ± 0.02
0.19 ± 0.01
Qcycle
F×Γ×T1/2 [s−1]
0.30 ± 0.00
0.34 ± 0.02
0.31 ± 0.01
0.39 ± 0.01
36
Table S5. Qcycle and Γ for the EABs under periodic polarization.
37
Qcycle is the amount of charge collected per surface of electrode during one respective half-
38
period of polarization (turnover condition). Γ is the surface coverage in electrically connected
39
electron carriers of the EABs (obtained under non-turnover conditions). All values are
40
averaged for the four last days of recording, when a quasi-steady state was reached. F is
41
Faraday constant. The ratio [Qcycle/(F. Γ.T1/2)] therefore corresponds to the amount of the EAB
42
charge capacity (in coulomb storable per EAB) discharged by the EAB into the anode during
43
1 s of average discharge. Interestingly, this amount was always comprised between 0.3 and
44
0.39 for these EABs under periodic polarization.
45
46
8
47
Fig. S6. Growth of mature EABs for the preliminary experiment.
48
Chronoamperograms of the 8 electrodes grown at − 0.1 V vs. Ag/AgCl with initially 24 mM
49
acetate as substrate. All 8 electrodes showed good reproducibility of current generation. The
50
catholyte was replaced daily to avoid an excessive pH increase which might degrade the ion
51
exchange membrane. Anolyte was refreshed once after current decayed below ~20% of
52
maximum. Different charging time measurements were performed at the current maximum in
53
the second batch phase (~ 250 h).
54
55
56
57
58
59
60
9
61
Fig. S7. Electrochemical responses of mature EABs with different tc.
62
(a) Evolution of OCP during a 60 s charge under turnover condition; (b) Discharge
63
chronoamperograms for tc = 5 s (orange circle) and tc = 300 s (red circle).
64
During the charging time interval, the OCP of the electrode quickly decreases because of the
65
charge accumulation at the vicinity of the electrode due to reduction of the charge carriers (a).
66
Discharge currents after 5s or 300 s of charging decayed sharply, which reflected a quick
67
release of stored electrons. The minimum current of transient curves after 300 s charge is
68
smaller than final current after 60 s discharge, which suggests a longer recovery time of
69
metabolic activity after longer charging time.
70
71
10
72
Fig. S8. Evolution of OCPf and estimated fraction of redox cofactors reduced at the electrode
73
interface (CR/CT) with different tc.
74
Performed on mature EABs previously grown under continuous polarization. The proportion
75
of redox cofactors reduced at the electrode interface was estimated via the Nernst equation
76
assuming a single redox cofactor of E1/2 = − 0.35 V vs. Ag/AgCl (Fig. S2).
77
11
78
Fig. S9. Cyclic voltammograms of mature EABs for preliminary experiment.
79
The turnover (a) and nonturnover (b) CVs were recorded at 50 mV s−1. The turnover CV
80
presented a typical sigmoidal shape. The apparent midpoint potential (E1/2) of the mature EAB
81
was determined by the first derivative and half-wave potential of the turnover CV at − 0.35 V
82
vs. Ag/AgCl, which is consistent with previous studies (Virdis et al. 2016; Zhang et al. 2017).
83
84
12
85
Fig. S10. Schematized, conjectured concentration profiles for the reduced form of the redox
86
charge carrier (Cred) during a transition from a polarization at − 0.1 V vs Ag/AgCl (purple) to
87
a charge under OCP (under turnover conditions). The scheme assumes a constant
88
concentration along the EAB = [Cred] + [Cox], where Cox represents the oxidized form of the
89
charge carrier (Schrott et al.).
90
The 𝑥 axis shows the normal distance from the electrode surface, within the EAB. At t ≤ 0
91
(polarization), a steady-state concentration gradient is expected (purple line), with all the
92
redox cofactors oxidized at the electrode surface x = 0 (plateau current of the polarization
93
curve i.e. electron transport limitation). Once the polarization is switched to OCP, the electron
94
sink (anode) is turned off. With increasing time of charge, the concentration profiles should
95
flatten (by electron diffusion, initially toward the electrode interface) and increase (by
96
microbial metabolic activity along the EAB, providing new electrons to the redox matrix). It
97
can be reasonably conjectured that the fraction of reduced cofactor at x = 0 (estimated by the
98
Nernst equation) gives an estimation of a minimum state of charge of the EAB.
99
100
13
101
Fig. S11. Representative transient current at the start of discharge for the different periodic
102
polarizations.
103
The corresponding half-periods of charge/discharge are stated on their respective panel. The
104
CAs were recorded after 220 h of growth under intermittent operation.
105
Note that the local minimum in the transient current for the EAB with T1/2 of 60 s occurs
106
before the one for the EAB with T1/2 of 300 s (1.2 s vs. 1.9 s, respectively). This suggests a
107
longer recovery of the microbial (electro)activity when the microbial metabolism was
108
interrupted for a longer time (once the EAB is fully charged).
109
110
14
111
Fig. S12. Recording the apparent steady-state catalytic current for EABs grown under
112
periodic polarization.
113
The steady-state catalytic current of each electrode was recorded daily, after 300 s of
114
chronoamperometry at − 0.1 V vs. Ag/AgCl. The measurement allows an objective
115
comparison of the electroactivity of the different EABs because the initial current of discharge
116
is transient. The respective periodic polarizations are restarted at the end of the recording. The
117
figure shows one representative chronoamperogram for each kind of EAB recorded after 220
118
h of growth.
119
15
120
Fig. S13. Charge surface coverage estimation (Γ, in mole- cm−2).
121
The amount of charges storable in the EABs was determined under nonturnover conditions by
122
background subtracted chronocoulometry monitoring a full EAB discharge at − 0.1 V (after
123
having fully charged the EAB at − 0.6 V vs. Ag/AgCl for 300 s) (Zhang et al. 2017).
124
16
125
Fig. S14. Representative non-turnover CVs of EABs grown under continuous (control) and
126
periodic polarization.
127
Recorded at 5 mV s−1 at the end of the experiment (10 days). The substantial increase of the
128
redox features for EAB-1s and EAB-10s already suggests a higher concentration of charge
129
carriers and/or a faster electron transport within these EAB when compared to the control.
130
The opposite is true for EAB-60s and EAB-300s.
131
17
132
133
134
135
136
Fig. S15. Proportional relationship between jss and Γ (top); and between jss and C (bottom).
137
Values of jss are the one averaged for the four last days of operation, when performances were
138
stabilized. Values of Γ and C were obtained at the end of the experiment (10 days).
139
This first order kinetics with respect to C suggests that the concentration of charge carriers
140
was a limiting factor for the production of the biocatalytic current. Assuming an analogy with
141
a model of redox conduction for oxidoreductase entrapped in redox polymers (Bartlett and
142
Pratt 1995), and in our particular case of substrate saturation, this would mean that the main
143
rate determining step for the value of jss was either:
144
(i) the electron transfer from the biocatalyst to the extracellular redox matrix. In this case
145
most redox cofactors along the EAB are almost exclusively oxidized since the
146
18
kinetics of electron transport along the EAB would be much faster in comparison (flat
147
steady state concentration profile where [Cred] ~ 0 and [Cox] ~ C). This case
148
corresponds to the kinetics regime I described by Bartlett and Pratt in their model
149
(Bartlett and Pratt 1995). An experimental observation of the case (i) could be the thin,
150
still growing EAB observed by Liu and Bond where most c-type cytochrome were
151
recorded as oxidized along the EAB (by spectroelectrochemistry) (Liu and Bond
152
2012).
153
(ii) A second case is when the current is simultaneously limited by the kinetics of electron
154
transfer from the biocatalyst to the redox matrix and by the kinetics of electron
155
transport along the redox matrix, both mechanisms being interrelated. In this case the
156
concentration profile of the reduced cofactor substantially increases with the normal
157
distance from the electrode (see above a representative schematized example: purple
158
curve in Figure S10, or a real measurement across a thick, mature EAB (Liu and
159
Bond 2012) performed by Liu and Bond). This case corresponds to the kinetics
160
regime II described by Bartlett and Pratt in their model (Bartlett and Pratt 1995).
161
Under substrate saturation and in a well-buffered medium, the case (i) would imply a
162
continuous growth of the EAB thickness (until it would finally reach the case (ii)!). Since
163
EABs typically reach a maximal thickness, and since a substantial redox gradient has been
164
commonly observed across mature EABs, we believe that the case (ii) appears more
165
reasonable for mature EABs having reached their maximum current. That said, this very short
166
discussion was based on an analogy with a “simple” model of redox conduction for hydrogels
167
where biocatalysts and redox cofactors are homogeneously distributed. Anodic EABs
168
certainly show another level of complexity, including the very possible implication of pili in
169
the electron transport processes.
170
19
171
Fig. S16. Representative Cottrell plots for the discharge of the different EABs.
172
The charge transport parameters (C × Dapp1/2) were extracted from Cottrell slopes, as
173
previously described (Zhang et al. 2017).
174
20
175
Fig. S17. SDS-PAGE gel (15 % acrylamide) with denatured DNA samples for each EAB.
176
The column on the right contains the protein markers (NZYColour Protein Marker II from
177
NZYTECH). The samples are control EABs from reactor 1 (A) and reactor 2 (B); EABs-1s
178
(C), EABs-10s (D), EABs-60s (E) and EABs-300s (F). Gel was revealed with heme staining
179
to show c-type cytochromes.
180
21
181
182
Fig. S18. Ratio of heme over total protein content (top) and optical absorption spectra of
183
heme from the different EABs (bottom).
184
The molar absorption coefficient was ε410nm = 125,000 M−1 cm−1 in 100 mM sodium
185
phosphate buffer, pH = 7.6.
186
22
187
Fig. S19. Calibration curves of total protein.
188
By using Pierce BCA Protein Assay Kit, the working range is from 5 to 250 µg mL−1 (SD
189
stands for standard deviation with n = 3).
190
The working reagent was prepared by mixing part A (50 parts) with part B (1 part) from BCA
191
reagent kit. Then 80 µL of sample or bovine serum albumin (BSA) as standard was added into
192
1.6 mL working reagent and mixed well. A water bath was used to heat samples at 60 °C for
193
30 min (enhanced procedure of working range 5 ~ 250 µg mL−1). After cooling down to room
194
temperature, the absorbance of all samples and standards were measured at 562 nm.
195
196
197
198
199
200
201
202
23
Table S20. Morphologic characteristics of the EABs.
203
‘Mushroom’ outer diameter means the maximal length of mushroom-like structure, while
204
inner diameter represents the maximum length of the inside hole.
205
206
Control 1
Control 2
EABs-1s
EABs-10s
EABs-60s
EABs-300s
Average
thickness / µm
36.7 ± 0.6
39.1 ± 4.8
37.6 ± 0.8
35.1 ± 0.0
24.9 ± 1.2
11.2 ± 0.3
‘Mushroom’
outer diameter
size / µm
nd
nd
32 ± 4
25 ± 2
69 ± 9
37 ± 3
‘Mushroom’
inner diameter
(hole) size / µm
nd
nd
nd
nd
17 ± 5
11 ± 1
24
207
Fig. S21. Microbial community composition of EABs at the end of the 10 days operation.
208
209
DNA extraction was performed by using the FastPrep method as previously described
210
(Vaidya et al. 1998). Biofilms were scratched from the electrode surface and stored in a
211
Lysing Matrix E tube (Qbiogene, Alexis Biochemicals, Carlsbad, CA), cell pellets were re-
212
suspended in lysis buffer containing 100 mM Tris/HCl (at pH 8.0), 100 mM EDTA, 100 mM
213
NaCl, 1% (w/v) polyvinylpyrrolidone and 2% (w/v) sodium dodecyl sulfate. Cells were lysed
214
using 0.2 mL beads of 0.1 mm size in a Fast Prep-96 homogenizer (40 s at 1600 rpm, twice).
215
After 1min centrifugation at 18,000 × g, the supernatant was washed using phenol/chloroform
216
(1:1) and chloroform. Next, DNA was precipitated with 1:10 volume sodium acetate (3M) and
217
1 volume isopropyl alcohol at −20 °C more than 1 h. Samples were centrifuged and washed
218
with 80% ethanol, then resuspended in 50 μL of Milli-Q water. The quality and quantity of
219
DNA samples were assessed by polymerase chain reaction (PCR) using the primers described
220
below. PCR products were then separated by electrophoresis on 1.2% agarose gels.
221
The V3–V4 regions of the bacterial 16S rRNA genes was sequenced using the Illumina
222
platform (LGC Genomics GmbH, Berlin, Germany) by using 2 × 300 bp paired-end reads and
223
the primers 341F (5’-NNNNNNNNTCCTACGGGNGGCWGCAG) and 785R (5’-
224
NNNNNNNNTGACTACHVGGGTATCTAAKCC) as previously described (Stewardson et
225
al. 2015). Each PCR included DNA extract (~ 5 ng), forward and reverse primer (~15 pmol
226
25
for each) and MyTaq buffer (20 μL containing 1.5 units MyTaq DNA polymerase (Bioline)
227
and 2 μL of BioStabII PCR Enhancer) (Prokopenko et al. 2013). 8-nt barcode sequence was
228
performed for both forward and reverse primers of each sample.
229
230
PCR procedures included: 1) 2 min pre-denaturation at 96 °C; 2) 30 cycles × (15 s at 96 °C,
231
30 s at 50 °C and 60 s at 72 °C). DNA concentration of the amplicons of interest were
232
estimated by gel electrophoresis. Amplicon DNA (~20 ng) of each sample were pooled for an
233
amount of 48 samples each carrying different barcodes. Another 5 cycles were amplified for
234
the low yield PCRs. One volume AMPure XP beads (Agencourt) was used to clean primer
235
dimer and remove other small mispriming products. An additional purification on MinElute
236
columns (Qiagen) were used to purify the amplicon pools. Each purified amplicon pool DNA
237
(~ 100 ng) was taken for Illumina libraries construction by using the Ovation Rapid DR
238
Multiplex System 1-96 (NuGEN). Illumina libraries were pooled and size selected with
239
preparative gel electrophoresis. Sequencing was done on an Illumina MiSeq using v3
240
Chemistry (Illumina). The Mothur community pipeline (Schloss et al. 2009) was used to
241
analyze and cluster 16S rRNA gene sequences into operational taxonomic units (OTUs) as
242
previously reported (Andersen et al. 2015). The analysis procedure included the following
243
processes: clipping 16S rRNA sequences from primers; combining and turning into forward
244
and reverse primer orientation sequences of the fragments (after removing the primer
245
sequences); removing the wrong size sequences and making them unique; aligning the
246
sequences with a V3–V4 customized SILVA database which were not match or overhung;
247
removing chimera with UCHIME software (Edgar et al. 2011). Then sequences of
248
taxonomical classification and non-bacterial removal were executed with Silva database v.123.
249
OTUs were picked up by clustering (97%) identity level using cluster split method.
250
26
251
Fig. S22. Fraction of Geobacter determined by Illumina sequencing of different EABs.
252
The microbial communities were characterized for only one EAB per period, except for
253
controls (CT), were n = 2. No trend was observed with respect to the half-period of
254
polarization.
255
256
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257
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