Cellular membranes exist across all domains of life, essentially acting as a barrier that separates the conditions of life from the non-living environment. In gram-negative bacteria, such as Escherichia coli, the cell envelope is comprised of an outer membrane (OM), a peptidoglycan layer (ca. 35–55 nm) and an inner membrane (IM). The core structural component to each membrane is the lipid bilayer, which is electrically insulating and impermeable to most ions and polar molecules. Membrane proteins are the other major component in microbial membranes, conferring function to the membrane and enabling the passive and active transport of ions, molecules and water. Critical biological processes, such as energy generation and molecular sensing, are inherently electronic—driven by the flow of electrons and ions across the membranes of cells. Thus, modification of the transmembrane flux of ions or electrons may enable manipulation of a wide-range of intracellular biological processes pertinent to biotechnological applications.
Conjugated oligoelectrolytes (COEs) are a class of small molecules designed for membrane modification. The structure of COEs are described by a pi- conjugated, phenylenevinylene backbone of a few repeat units tethered at the two terminal ends by ionic functionalities. COEs have been demonstrated to spontaneously intercalate and align in lipid bilayers, thereby allowing modification of membrane properties and function of cells in bioelectrochemical systems. One COE, namely DSSN+, has been applied in a variety of microbial electronic devices utilizing Yeast, E. coli, Shewanella oneidensis, and even naturally occurring microorganisms in wastewater to improve current generation. It is generally acknowledged that ability of COEs to intercalate into microbial membranes is paramount for increasing charge transfer in bioelectrochemical systems, however the specific mechanism of action is not well understood. Previous investigations on the mechanism for improved current in S. oneidensis suggest that DSSN+ amplifies the native biological electron transfer pathway. However, this suggested mechanism does not universally apply across microbial species. For example, COEs have been demonstrated to increase in current and power generation in E. coli microbial fuel cells and this organism lacks a native extracellular electron transfer pathway.
A unifying mechanism by which COEs improve charge transfer processes and modify microbial membranes is the underlying motivation for this work. First, COEs varying in length and structural features are compared with respect to their association with E. coli. Quantification of COEs associated with the cell reveals a morphologically impossible amount, approaching or surpassing a 1:1 lipid:COE ratio, indicating association is not exclusive to membrane intercala- tion. Using zeta potential measurements on COE modified cells, it is determined that COEs are able to tune cell surface charge. Second, considering permeabilization as one possible mode for improved bioelectrochemical performance, the effect of DSSN+ on the permeability of the inner and outer membrane of E. coli is examined, revealing a plausible explanation. Lastly, performance of E. coli in bioelectrochemical systems is examined while taking these effects on membrane properties into consideration. An ultimate hypothesis is proposed, that the combined effects of COEs on membrane properties are the underlying cause for the increased current in E. coli. A better understanding of the effects of COEs on microbial membrane properties can thus inform the molecular design of future COEs and uncover potential new areas of application.