Oxygen Tension and Riboflavin Gradients Cooperatively Regulate the Migration of Shewanella oneidensis MR-1 Revealed by a Hydrogel-Based Microfluidic Device

Beum Jun Kim,Sebastian Jusuf,Tiffany Kuo,Largus T Angenent,Mingming Wu,Michaela A Teravest,Injun Chu

doi:10.3389/fmicb.2016.01438

Beum Jun Kim, Sebastian Jusuf + Show 5 more

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https://doi.org/10.3389/fmicb.2016.01438

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Abstract

Shewanella oneidensis is a model bacterial strain for studies of bioelectrochemical systems (BESs). It has two extracellular electron transfer pathways: (1) shuttling electrons via an excreted mediator riboflavin; and (2) direct contact between the c-type cytochromes at the cell membrane and the electrode. Despite the extensive use of S. oneidensis in BESs such as microbial fuel cells and biosensors, many basic microbiology questions about S. oneidensis in the context of BES remain unanswered. Here, we present studies of motility and chemotaxis of S. oneidensis under well controlled concentration gradients of two electron acceptors, oxygen and oxidized form of riboflavin (flavin+), using a newly developed microfluidic platform. Experimental results demonstrate that either oxygen or flavin+ is a chemoattractant to S. oneidensis. The chemotactic tendency of S. oneidensis in a flavin+ concentration gradient is significantly enhanced in an anaerobic in contrast to an aerobic condition. Furthermore, either a low oxygen tension or a high flavin+ concentration considerably enhances the speed of S. oneidensis. This work presents a robust microfluidic platform for generating oxygen and/or flavin+ gradients in an aqueous environment, and demonstrates that two important electron acceptors, oxygen and oxidized riboflavin, cooperatively regulate S. oneidensis migration patterns. The microfluidic tools presented as well as the knowledge gained in this work can be used to guide the future design of BESs for efficient electron production.

Highlights

Certain species of microbes (e.g., Geobacter spp.; Shewanella spp.; Pseudomonas spp.) have been found to transfer electrons from organic sources to extracellular electrodes, generating an electric current, which is the basis for a bioelectrochemical system (BES; Logan et al, 2006; Richter et al, 2008)
Because S. oneidensis transfer electrons both via secreted flavin as well as direct contact of the cell with the electrode, we argue that cell migration pattern within a BES critically controls the spatial distribution of bacteria within a BES, regulating electron transfer efficiency
The aerotactic tendency, measured by chemotactic migration coefficient (CMC) supports that E. coli shows aerotaxis behavior, with 0.028 ± 0.002 vs. 0.005 ± 0.002

Summary

Introduction

Certain species of microbes (e.g., Geobacter spp.; Shewanella spp.; Pseudomonas spp.) have been found to transfer electrons from organic sources to extracellular electrodes, generating an electric current, which is the basis for a bioelectrochemical system (BES; Logan et al, 2006; Richter et al, 2008). Directed Migration of Shewanella oneidesis transfer process at the anode, which involves complex molecular and cellular transport (Figure 1). Electron mediators (e.g., riboflavin or phenazine) secreted by the microbes assist the electron transfer process at the anode. Shewanella oneidensis MR-1 is a model microbial strain for BES, because it transfers electrons extracellularly via two paths: (1) a mediated extracellular electron transfer (MEET) using endogenously produced riboflavin; and (2) a direct extracellular electron transfer (DEET) via membrane bound cytochrome families (Figure 1) (Marsili et al, 2008; Li et al, 2012). Note that S. oneidensis is reported to produce reduced flavins via the Mtr pathway (Li et al, 2012)

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