Living electronics: Coupling S. oneidensis MtrC-SpyTag cells to SpyCatcher-functionalized electrodes for direct electron transfer

This work set out to establish an easily applicable system to improve a broad range of bioelectronic devices using the SpyTag-SpyCatcher crosslinking system together with one of the model organisms for extracellular electron transport, Shewanella oneidensis. Therefore, the surface-displayed c-type cytochrome MtrC was equipped with an accessible SpyTag and coupled to SpyCatcher-functionalized surfaces. A transposon screen followed by nanopore sequencing was conducted in order to identify integration positions which facilitate MtrC functionality while the SpyTag is surface accessible. Three integration positions (W314, Y417, T603) were chosen for further characterization. Expression of the MtrC-SpyTag constructs in a S. oneidensis strain lacking all outer membrane cytochromes restored the ability to reduce an extracellular electron acceptor. Two of the three strains reached reduction rates at the wildtype MtrC level proving that the integrated SpyTag does not hamper extracellular electron transfer. In vivo, all three constructs showed significantly better binding properties to SpyCatcher functionalized magnetic beads than the wildtype MtrC control. The two most promising candidates were coupled to conductive, magnetic gold nanoparticles and directed towards a screen-printed electrode showcasing how MtrC-SpyTag expression can improve bioelectronic devices. A significantly higher charge transfer compared to the wildtype control was reached in linear sweep voltammetry and chronoamperometry experiments. Moreover, a shift towards direct electron transfer was observed which reduces the problem of redox shuttle washout in flowthrough systems. Direct binding of the cells to SpyCatcher functionalized electrodes enabled robust current production even after thorough washing of the electrodes while control cells failed to produce current under these conditions. The versatile SpyCatcher toolbox can be used together with the here reported strains to eliminate bottlenecks in bioelectronics like poor biofilm formation or the production of an insulating extracellular polymer matrix.