Self-assembled electroactive bacterial network Novel bacterium biofilms

The ability of certain bacterial strains to relay electrons to extracellular acceptors so as to foster cell respiration has intrigued researchers for the last two decades [1]. Such bacteria, called electroactive bacteria (EAB), can exist even in hostile environments and possess unique biophysical properties [2]. They are classified as electrogenic bacteria or electrotrophic bacteria depending upon their ability to either donate or consume electrons respectively. They play a vital role in the development of self-sustainable bio-batteries, treatment of wastewater, production of hydrogen and other value added chemicals besides bioremediation [3].

Microbial Fuel Cells (MFCs) are unique systems which exploit the intrinsic characteristics of electrogenic bacteria to donate electrons to conductive electrodes. These bacteria grow on wastewater and generate electrons by degrading the organic carbon present in the wastewater. Hence, they facilitate simultaneous wastewater treatment as well as generating power [4]. These systems typically consist of a biocompatible anode facilitating the adhesion of electrogenic bacteria, a separator and, a cathode for electron reduction. In most cases, this biofilm formation on the surface of electrodes is self-assembled. One of the major goal of researchers is to maximize the extracellular electron transfer by optimizing the bacteria-anode adhesion so that stable and enhanced power generation can be attained using MFCs [5].

Electrogenic bacteria perform the extracellular electron transfer either with the help of certain unique outer cell membranes proteins or through conductive pili called the “bacterial nanowires” or electron shuttling molecules [6]. Among them, bacterial nanowires promote long-range electron transport across the bacterial biofilm by linking various layers of the conductive biofilm matrix. In such cases, considering that each cell contributes to electron donation, the amount of active bacteria in the biofilm decides the net electron flux and thus current generation of the system. Thus, researchers have been actively screening potential strains which can form dense and highly conductive biofilms.

Screening of electrogenic bacteria can be done by various methods including MFCs, photometric assays, dye reduction assays etc. [7], [8]. Among these methods, MFCs can give valuable insights on the electrochemical properties of bacteria and mechanisms of electron transfer besides quantifying their net electron generation capability. However, the major drawback of these conventional MFCs is that time taken for the formation of self-assembled biofilm in such devices is of the order of days-to-months. This can be overcome by using miniature or micro-fabricated systems which facilitate rapid bacterial colonization on electrodes thus providing quick and precise results [9].

Our research focusses on the development of portable and miniaturized electrochemical cells to screen electrogenic strains. We have developed a custom-fabricated, scaled-down MFC which can give an accurate estimate of the electrogenic potential of microbes within 6?h. We gather cultures from various ecological niches to identify the most potent electrogen using the miniature cell and we further compare their performance with standard electrogenic bacteria. The portable device consisted of carbon felt anodes which confer high electrical conductivity, chemical inertness, biocompatibility and high surface area for bacterial adhesion. Nonetheless, with carbon felt being optically non-transparent and porous, conventional microscopy techniques fail to probe the bacterial colonization across the fibres of carbon felt. Thus Scanning Electron Microscopy (SEM) is the most pertinent alternative to investigate the bacterial attachment to the anode.

The SEM image shown on this issue’s cover corresponds to the self-assembled biofilm of a novel bacterium, (yet to be identified) on the surface of carbon felt anode of the miniature electrochemical cell. The isolate was cultured in the laboratory in a synthetic, defined nutrient solution and injected into the device to promote colonization. Our experimental results demonstrated that this particular isolate had an incredibly large electron transfer rate as compared to the standard electrogenic strains. This can be attributed to high biofilm density on the anode surface. To evaluate this, the anode was examined using a ZEISS scanning electron microscope. It is observed that the fibres of carbon felt were completely encased by the bacteria and significant microbial population is found plugging into the core of the porous felt matrix. This evidence indicates that increased bacterial adhesion might be responsible for higher current generation capability of the strain.

We envisage that our endeavours will help in identifying potent bacterial strains with high electrogenic potential so that tailor-made microbe-anode interfaces can be designed for the generation of electricity.

Acknowledgements

The author is thankful to Indian Institute of Technology, Kharagpur and the technical assistance provided by the Central Research Facility of the Institute.

Self-assembled electroactive bacterial network Novel bacterium biofilms

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