(177a) On the Influence of Outer Membrane Cytochromes on Cell Polarizability

On the Influence of Outer Membrane Cytochromes on Cell Polarizability

Qianru Wang¹, Pei Zhang¹, A-Andrew Jones III¹, Liwei Lin² and Cullen R. Buie¹

¹Department of Mechanical Engineering, Massachusetts Institute of Technology

²Department of Mechanical Engineering, University of California, Berkeley

This work elucidates for the first time the strong correlation between extracellular electron transfer (EET) facilitated by outer membrane cytochromes and cell envelope polarizability. EET is a crucial cell phenotype for applications including environmental remediation, microbial fuel cells (MFC), and microbial electrosynthesis. Expression of outer membrane c-type cytochromes plays an important role in bacteria respiration and EET. Although various redox active anode-reducing cytochromes critical for EET in Geobacter sulfurreducens (G. sulfurreducens) have been investigated^1-3, and microfluidic systems have been explored to measure cellular electrical properties^4,5, the correlation between the cellular physiological properties and their electrical phenotype has not been revealed. In this work, analysis of wild type G. sulfurreducens and cytochrome deletion mutants show that expression of outer membrane cytochromes, which are responsible for EET, influence cell envelope polarizability as measured in low cost microfluidic systems employing dielectrophoresis (DEP). The strong correlation between EET and cell surface polarizability can be exploited to identify novel electrochemically active bacteria, with broad implications for biotechnology, bioremediation, and biomedical applications.

We find that the expression of periplasmic and outer membrane bound c-type cytochromes has a significant effect on G. sulfurreducens surface polarizability. Generally speaking, strains expressing larger amounts of c-type cytochromes display higher polarizability. However, depending on the specific function of the c-type cytochrome, the various genes explored influence cell envelope polarizability differently. C-type cytochromes important for oxidation/reduction reactions, but not actively involved in electron transfer, appear not to influence cell envelope polarizability. Furthermore, simultaneous deletion of multiple outer membrane bound c-type cytochrome genes doesn’t result in linear reduction of cell polarizability due to the strain's ability to adapt increased expression of other cytochrome genes. In conclusion, for the first time the electrochemical phenotype of cells is definitively correlated with cell polarizability. This work demonstrates the power of microfluidic techniques such as DEP for microbial biotechnology. Future work in this area could extend to the development of novel microfluidic tools employing DEP to identify or evolve microbes for bioelectrochemical systems and other applications dependent upon EET.

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