Characterization of electrochemically active bacteria utilizing a high-throughput voltage-based screening assay.

Metal reduction assays are traditionally used to select and characterize electrochemically active bacteria (EAB) for use in microbial fuel cells (MFCs). However, correlating the ability of a microbe to generate current from an MFC to the reduction of metal oxides has not been definitively established in the literature. As these metal reduction assays may not be generally reliable, here we describe a four- to nine-well prototype high throughput voltage-based screening assay (VBSA) designed using MFC engineering principles and a universal cathode. Bacterial growth curves for Shewanella oneidensis strains DSP10 and MR-1 were generated directly from changes in open circuit voltage and current with five percent deviation calculated between each well. These growth curves exhibited a strong correlation with literature doubling times for Shewanella indicating that the VBSA can be used to monitor distinct fundamental properties of EAB life cycles. In addition, eight different organic electron donors (acetate, lactate, citrate, fructose, glucose, sucrose, soluble starch, and agar) were tested with S. oneidensis MR-1 in anode chambers exposed to air. Under oxygen exposure, we found that current was generated in direct response to additions of acetate, lactate, and glucose.

Simultaneous analysis of physiological and electrical output changes in an operating microbial fuel cell with Shewanella oneidensis.

Changes in metabolism and cellular physiology of facultative anaerobes during oxygen exposure can be substantial, but little is known about how these changes connect with electrical current output from an operating microbial fuel cell (MFC). A high-throughput voltage based screening assay (VBSA) was used to correlate current output from a MFC containing Shewanella oneidensis MR-1 to carbon source (glucose or lactate) utilization, culture conditions, and biofilm coverage over 250 h. Lactate induced an immediate current response from S. oneidensis MR-1, with both air-exposed and anaerobic anodes throughout the duration of the experiments. Glucose was initially utilized for current output by MR-1 when cultured and maintained in the presence of air. However, after repeated additions of glucose, the current output from the MFC decreased substantially while viable planktonic cell counts and biofilm coverage remained constant suggesting that extracellular electron transfer pathways were being inhibited. Shewanella maintained under an anaerobic atmosphere did not utilize glucose consistent with literature precedents. Operation of the VBSA permitted data collection from nine simultaneous S. oneidensis MR-1 MFC experiments in which each experiment was able to demonstrate organic carbon source utilization and oxygen dependent biofilm formation on a carbon electrode. These data provide the first direct evidence of complex cellular responses to electron donor and oxygen tension by Shewanella in an operating MFC at select time points.

The utility of Shewanella japonica for microbial fuel cells.

Shewanella-containing microbial fuel cells (MFCs) typically use the fresh water wild-type strain Shewanella oneidensis MR-1 due to its metabolic diversity and facultative oxidant tolerance. However, S. oneidensis MR-1 is not capable of metabolizing polysaccharides for extracellular electron transfer. The applicability of Shewanella japonica (an agar-lytic Shewanella strain) for power applications was analyzed using a diverse array of carbon sources for current generation from MFCs, cellular physiological responses at an electrode surface, biofilm formation, and the presence of soluble extracellular mediators for electron transfer to carbon electrodes. Critically, air-exposed S. japonica utilizes biosynthesized extracellular mediators for electron transfer to carbon electrodes with sucrose as the sole carbon source.