Research ArticleAPPLIED SCIENCES AND ENGINEERING

Microfluidic dielectrophoresis illuminates the relationship between microbial cell envelope polarizability and electrochemical activity

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Science Advances  11 Jan 2019:
Vol. 5, no. 1, eaat5664
DOI: 10.1126/sciadv.aat5664
  • Fig. 1 DEP phenotyping of G. sulfurreducens.

    (A) 3DiDEP microfluidic device with an array of multiple microchannels. A DC potential difference increasing linearly with time at 1 V/s was applied across the channel. Credit: Qianru Wang, MIT. (B) Magnified view of the microchannel highlighting the constricted area. (C) Schematic depicting the 3DiDEP trapping principle. Bacteria near the constriction are immobilized when the DEP force (Embedded Image), which is proportional to Embedded Image, is balanced by drag forces due to the background electroosmotic flow (Embedded Image) and electrophoresis (Embedded Image). The magnitude distribution of the x component of Embedded Image is illustrated in the background color scale (dark red indicates higher values). (D) Schematic showing that G. sulfurreducens c-type outer-membrane cytochromes mediate EET. (E) Measured trapping voltage [the threshold applied voltage at the onset of 3DiDEP trapping depicted in (C)] was plotted against the ratio of DEP mobility (μDEP) to the magnitude of linear electrokinetic mobility (μEK) of WT G. sulfurreducens DL-1, DL-1 inoculated in an MFC anode for 24 and 31 days, and various indicated cytochrome-deletion mutants. A significant difference (P < 0.05) was found between data groups isolated by dashed circles using a Kruskal-Wallis test. The black line indicates the inverse relationship between the ratio |μDEPEK| and the applied voltage.

  • Fig. 2 G. sulfurreducens cell polarizability is positively correlated with EET capacity.

    (A) Linear electrokinetic mobility (mean ± SD), μEK, of the studied nine strains of G. sulfurreducens. (B) DEP mobility, μDEP, of the studied nine strains of G. sulfurreducens. Pairwise comparison using two-sample t test (two-tailed) shows significant difference (P < 0.03) between groups not sharing letters (italic bold). (C to E) Box-whisker plots of cell major semi-axis (C), minor semi-axis (D), and the ratio of Perrin friction factor to the square of cell short semi-axis ξ/b2 (E) by ellipsoidal fit for the nine investigated G. sulfurreducens strains indicate median and interquartile ranges (IQRs). The whiskers extend to 1.5 IQR below the 25th percentile and above the 75th percentile, respectively. Blank dots indicate the outliers. Asterisks indicate significant difference (P < 0.01) compared to the control (WT DL-1) by a Kruskal-Wallis test. The numbers of measured cells (n) are 150, 100, 100, 244, 100, 445, 238, 50, and 100, respectively, following the order in (A). Inserted plot is a high-magnification micrograph showing the ellipsoidal fit of a WT DL-1 bacterium. (F) G. sulfurreducens polarizability, represented by the Clausius-Mossotti factor (κCM), of the nine investigated strains (left y axis), as well as the current density (blue circles, right y axis) generated by G. sulfurreducens grown in an MFC. Italic bold letters above the bars show the result of pairwise comparison using two-sample t test (two-tailed) with the following number of repeats: n = 3 (WT DL-1, ΔomcBST), n = 4 (ΔomcB, ΔomcZ, ΔomcBS, ΔomcBEST, and ΔomcBESTZ), n = 5 (MFC, 24 days), and n = 7 (MFC, 31 days). A significant difference (P < 0.02) was found between groups not sharing letters. Colors in all panels correspond to the legend in Fig. 1E.

  • Fig. 3 DEP screening indicates positive correlation between S. oneidensis polarizability and EET activity.

    (A) Schematic of the Mtr EET pathway in the S. oneidensis cell envelope. (B) Fe(III) citrate reduction over time measured for S. oneidensis WT strain MR-1, strain deficient in expressing both MtrABC and MtrDEF EET conduits (ΔMtr), and ΔMtr complemented with indicated proteins. Error bars indicate the SD. (C) Polarizability of the five S. oneidensis strains corresponding to (B) grown with different electron accepters, namely, (i) 60 mM pure fumarate and (ii) 15 mM Fe(III) citrate supplemented with a small amount (10 mM) of fumarate. Growth conditions in (B) and (C) (ii) are identical. The box-whisker plots indicate median and IQRs, with whiskers extending to 1.5 IQR below the 25th percentile and above the 75th percentile, respectively. Black crosses indicate outliers. Multiple comparison test of group means using one-way analysis of variance (ANOVA) suggests a significant difference (P < 0.05) between groups labeled with different letters. Asterisk indicates a significant difference (P < 0.03, two-tailed t test) between the polarizability of iron-reducing S. oneidensis (ii) and that of its fumarate-reducing counterpart (i).

  • Fig. 4 E. coli introduced with EET pathways from S. oneidensis gains strong polarizability.

    (A) Polarizability of the E. coli strain transformed with an empty cytochrome c maturation (ccm) plasmid (control) and the strain cotransformed with S. oneidensis MtrABC EET conduit grown with 15 mM Fe(III) citrate and 10 mM fumarate. The electrogenic E. coli strain obtains significantly enhanced polarizability (P < 0.0001, two-tailed t test; n = 8) compared to the control. (B) Fe(III) citrate reduction over time measured for the control and the electrogenic E. coli strain. (C) Positive relationship between bacterial polarizability and iron reduction rate of the studied five S. oneidensis strains and two E. coli strains is indicated by a log fitting (dashed line) with a fitting goodness of R2 = 0.91. The iron reduction rate was derived by taking the slope of the linear portion of the Fe(III) citrate reduction curves.

  • Table 1 Strains used in this work.
    StrainRelative genotypeSource
    G. sulfurreducens strains
      WT DL-1G. sulfurreducens strain DL-1, WTLeang et al. (8)
      ΔomcBWT DL-1 strain without omcBLeang et al. (8)
      ΔomcZWT DL-1 strain without omcZNevin et al. (16)
      ΔomcBSWT DL-1 strain without omcB/omcSLeang et al. (8)
      ΔomcBSTWT DL-1 strain without omcB/omcS/omcTLeang et al. (8)
      ΔomcBESTWT DL-1 strain without omcB/omcE/omcS/omcTVoordeckers et al. (37)
      ΔomcBESTZWT DL-1 strain without omcB/omcE/omcS/omcT/omcZVoordeckers et al. (37)
    S. oneidensis strains
      WT MR-1S. oneidensis strain MR-1, WTCoursolle and Gralnick (10)
      ΔMtrΔmtrBmtrEmtrComcAmtrFmtrAmtrDdmsESO4360cctArecACoursolle and Gralnick (10)
      ΔMtr+MtrABCΔMtr strain carrying plasmid pmtrB/mtrC/mtrACoursolle and Gralnick (10)
      ΔMtr+MtrDEFΔMtr strain carrying plasmid pmtrE/mtrF/mtrDCoursolle and Gralnick (10)
      ΔMtr+vectorΔMtr strain carrying an empty vector pBBR-BBCoursolle and Gralnick (10)
    E. coli strains
      ccmE. coli strain C43 carrying ccmA-HJensen et al. (42)
      ccm+CymA/MtrABCStrain ccm cotransformed with cymAmtrCABJensen et al. (42)

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/1/eaat5664/DC1

    Fig. S1. The effect of cell shape on the Clausius-Mossotti factor.

    Fig. S2. MFC incubation time affects G. sulfurreducens polarizability.

    Fig. S3. S. oneidensis electrokinetics and cell morphology.

    Fig. S4. Electrogenic E. coli electrokinetics and cell morphology.

    Table S1. Summary of G. sulfurreducens c-type outer-membrane cytochromes in this study and their roles in EET.

    Movie S1. 3DiDEP immobilization of G. sulfurreducens.

    Movie S2. Measurement of linear electrokinetic mobility using particle image velocimetry.

    Section S1. Calculation of the Clausius-Mossotti factor for two-shelled ellipsoidal particles

    Section S2. Microfluidic 3DiDEP device

    References (4648)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. The effect of cell shape on the Clausius-Mossotti factor.
    • Fig. S2. MFC incubation time affects G. sulfurreducens polarizability.
    • Fig. S3. S. oneidensis electrokinetics and cell morphology.
    • Fig. S4. Electrogenic E. coli electrokinetics and cell morphology.
    • Table S1. Summary of G. sulfurreducens c-type outer-membrane cytochromes in this study and their roles in EET.
    • Legends for movies S1 and S2
    • Section S1. Calculation of the Clausius-Mossotti factor for two-shelled ellipsoidal particles
    • Section S2. Microfluidic 3DiDEP device
    • References (4648)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). 3DiDEP immobilization of G. sulfurreducens.
    • Movie S2 (.avi format). Measurement of linear electrokinetic mobility using particle image velocimetry.

    Files in this Data Supplement:

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