The different behaviors of three oxidative mediators in probing the redox activities of the yeast Saccharomyces cerevisiae

Microbial Electrochemical Systems: Principles, Construction and Biosensing Applications

Microbial electrochemical systems are a fast emerging technology that use microorganisms to harvest the chemical energy from bioorganic materials to produce electrical power. Due to their flexibility and the wide variety of materials that can be used as a source, these devices show promise for applications in many fields including energy, environment and sensing. Microbial electrochemical systems rely on the integration of microbial cells, bioelectrochemistry, material science and electrochemical technologies to achieve effective conversion of the chemical energy stored in organic materials into electrical power. Therefore, the interaction between microorganisms and electrodes and their operation at physiological important potentials are critical for their development. This article provides an overview of the principles and applications of microbial electrochemical systems, their development status and potential for implementation in the biosensing field. It also provides a discussion of the recent developments in the selection of electrode materials to improve electron transfer using nanomaterials along with challenges for achieving practical implementation, and examples of applications in the biosensing field.

Redox-Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

Microbial electrochemical systems in which metabolic electrons in living microbes have been extracted to or injected from an extracellular electrical circuit have attracted considerable attention as environmentally-friendly energy conversion systems. Since general microbes cannot exchange electrons with extracellular solids, electron mediators are needed to connect living cells to an extracellular electrode. Although hydrophobic small molecules that can penetrate cell membranes are commonly used as electron mediators, they cannot be dissolved at high concentrations in aqueous media. The use of hydrophobic mediators in combination with small hydrophilic redox molecules can substantially increase the efficiency of the extracellular electron transfer process, but this method has side effects, in some cases, such as cytotoxicity and environmental pollution. In this Review, recently-developed redox-active polymers are highlighted as a new type of electron mediator that has less cytotoxicity than many conventional electron mediators. Owing to the design flexibility of polymer structures, important parameters that affect electron transport properties, such as redox potential, the balance of hydrophobicity and hydrophilicity, and electron conductivity, can be systematically regulated.

Online-monitoring of biofilm formation using nanostructured electrode surfaces

The direct monitoring of biofilm formation enables valuable insights into the industrial processes, microbiology, and biomedical applications. Therefore, in the present study, nano-structured bioelectrochemical platforms were designed for sensing the formation of biofilm of P. aeruginosa along with monitoring its electrochemical/morphological changes under different stresses. Through the assay optimizations, the performances of different electrode modifiers such as reduced graphene oxide (rGO) nanosheets, hyperbranched chitosan nanoparticles (HBCs NPs), and rGO-HBCs nano-composite were tested to assess the influence of the electrode materials on biofilm progression. As a need for the anodic respiration, the bioelectrochemical responses of the adhered bacterial cells changed from a non-electrochemically active (planktonic state) to an electrochemically active (biofilm matrix) state. Our results demonstrated that electrode modifications with conductive nanostructured elements is highly sensitive and enable direct assay for the biofilm formation without any preachments. Consequently, the morphological changes in bacterial cell wall, upon switching from the planktonic state to the biofilm matrix were imaged using scanning electron microscopy (SEM), and the changes in cell wall chemical composition were monitored by the Energy Dispersive X-ray analysis (EDX). Thus, the designed microbial electrochemical system (MES) was successfully used to monitor changes in the biofilm matrix under different stresses through direct measurements of electron exchanges.

Synthesis of polypyrrole within the cell wall of yeast by redox-cycling of [Fe(CN)6](3-)/[Fe(CN)6](4-)

Yeast cells are often used as a model system in various experiments. Moreover, due to their high metabolic activity, yeast cells have a potential to be applied as elements in the design of biofuel cells and biosensors. However a wider application of yeast cells in electrochemical systems is limited due to high electric resistance of their cell wall. In order to reduce this problem we have polymerized conducting polymer polypyrrole (Ppy) directly in the cell wall and/or within periplasmic membrane. In this research the formation of Ppy was induced by [Fe(CN)6](3-)ions, which were generated from K4[Fe(CN)6], which was initially added to polymerization solution. The redox process was catalyzed by oxido-reductases, which are present in the plasma membrane of yeast cells. The formation of Ppy was confirmed by spectrophotometry and atomic force microscopy. It was confirmed that the conducting polymer polypyrrole was formed within periplasmic space and/or within the cell wall of yeast cells, which were incubated in solution containing pyrrole, glucose and [Fe(CN)6](4-). After 24h drying at room temperature we have observed that Ppy-modified yeast cell walls retained their initial spherical form. In contrast to Ppy-modified cells, the walls of unmodified yeast have wrinkled after 24h drying. The viability of yeast cells in the presence of different pyrrole concentrations has been evaluated.

Mediated bioelectrochemical system for biosensing the cell viability of Staphylococcus aureus

Staphylococcus aureus is one of the most dangerous human pathogens and is the cause of numerous illnesses ranging from moderate skin infections to life-threatening diseases. Despite advances made in identifying microorganisms, rapid detection methods for the viability of bacteria are still missing. Here, we report a rapid electrochemical assay for cell viability combining the use of double redox mediators and multiwall carbon nanotubes-screen printed electrodes (MWCNTs-SPE), ferricyanide (FCN) and 2,6-dichlorophenolindophenol (DCIP), which served as electron shuttle to enable the bacterial-electrode communications. The current originating from the metabolically active cells was recorded for probing the activity of the intracellular redox centers. Blocking of the respiratory chain pathways with electron transfer inhibitors demonstrated the involvement of the electron transport chain in the reaction. A good correlation between the number of the metabolically active cells and the current was obtained. The proposed assay has been exploited for monitoring cell proliferation of S. aureus during the growth. The sensitivity of the detection method reached 0.1 OD600. Therefore, the technique described is promising for estimating the cell number, measuring the cell viability, and probing intracellular redox center(s).

Screening the toxicity and biodegradability of petroleum hydrocarbons by a rapid colorimetric method

Crude oil and petroleum products have a wide variety of hazardous components with high toxicity and low biodegradability. Certain dyes change their colors by intercepting electron transfer reactions during the transformation processes. This study applied resazurin and 2,6-dichlorophenol-indophenol indicators for a rapid screening biodegradation capability and toxicity response to various petroleum products such as motor oil, diesel, gasoline, and phenol. Colorimetry tests were performed in test tubes, and the absorbance values were measured over time. We observed different discoloration profiles after degradation tests using Bacillus subtilis inoculum. Phytotoxicity assays were also performed to compare colorimetric screening assays with a conventional toxicity testing with plants (seed germination). The results indicated that biotransformation of oils can increase its overall toxicity. Intermediate byproducts can be formed through biodegradation and thereby increase the toxicity of oils. The assessment of acute toxicity has shown that phenol is extremely toxic to petroleum-biodegrading microbial communities. Low molecular-weight gasoline was considered biodegradable, but it also exhibited a high acute toxic effect, mainly due to its high solubility and the presence of more volatile compounds that can penetrate cells and potentially damage cellular structures.

Electrochemical detection of intracellular and cell membrane redox systems in Saccharomyces cerevisiae

Redox mediators can interact with eukaryote cells at a number of different cell locations. While cell membrane redox centres are easily accessible, the redox centres of catabolism are situated within the cytoplasm and mitochondria and can be difficult to access. We have systematically investigated the interaction of thirteen commonly used lipophilic and hydrophilic mediators with the yeast Saccharomyces cerevisiae. A double mediator system is used in which ferricyanide is the final electron acceptor (the reporter mediator). After incubation of cells with mediators, steady state voltammetry of the ferri/ferrocyanide redox couple allows quantitation of the amount of mediator reduced by the cells. The plateau current at 425 mV vs Ag/AgCl gives the analytical signal. The results show that five of the mediators interact with at least three different trans Plasma Membrane Electron Transport systems (tPMETs), and that four mediators cross the plasma membrane to interact with cytoplasmic and mitochondrial redox molecules. Four of the mediators inhibit electron transfer from S. cerevisiae. Catabolic inhibitors were used to locate the cellular source of electrons for three of the mediators.

Electrochemical sensing of hepatocyte viability

We investigated the use of amperometric and chronoamperometric methods with a double mediator system and screen-printed electrodes (SPEs) for the electrochemical sensing of hepatocyte viability. Cell counts were determined based on measuring cellular respiration via interaction of electroactive redox mediators. The oxidation currents of chronoamperometric measurement were proportional to the concentrations of ferrocyanide which was produced via interaction of cellular respiration, succinate and ferricyanide. The integrated oxidation charges increased linearly with the density of the cultured primary rat hepatocytes over a range of 1 × 10(5) to 5 × 10(5) cells per well (slope = 1.98 (±0.08) μC per 10(5) cells; R(2) = 0.9969), and the detection limit was 7600 (±300) cells per well based on S/N = 3. Each density of cells was cultured in triple replicates and individual cell samples were evaluated. The results of the cytotoxic effect of the chronoamperometric method are comparable to those of the tetrazolium-based colorimetric assay. The chronoamperometric method with ferricyanide and succinate mediators is an efficient, alternative method for assessing the viability of primary hepatocytes which can be completed in 20 min. Succinate did not provide an efficient electron shuttle between cytosolic respiratory redox activity of cancer cells and extracellular ferricyanide, an effect that may be useful for distinguishing hepatocarcinoma cells from healthy hepatocytes.

A disposable biofilm-modified amperometric biosensor for the sensitive determination of pesticide biotoxicity in water

A novel, disposable and sensitive whole cells biosensor based on permeabilizedS. cerevisiaehas been fabricated for the biotoxicity assessment of pesticides in wastewater.

Application of cyclic biamperometry to viability and cytotoxicity assessment in human corneal epithelial cells

The application of cyclic biamperometry to viability and cytotoxicity assessments of human corneal epithelial cells has been investigated. Electrochemical measurements have been compared in PBS containing 5.0 mM glucose and minimal essential growth medium. Three different lipophilic mediators including dichlorophenol indophenol, 2-methyl-1,4-naphthoquinone (also called menadione or vitamin K3) and N,N,N',N'-tetramethyl-p-phenylenediamine have been evaluated for shuttling electrons across the cell membrane to the external medium. Transfer of these electrons to ferricyanide in the extra cellular medium results in the accumulation of ferrocyanide. The amount of ferrocyanide is then determined using cyclic biamperometry and is related to the extent of cell metabolic activity and therefore cell viability. To illustrate cytotoxicity assessment of chemicals, hydrogen peroxide, benzalkonium chloride and sodium dodecyl sulfate have been chosen as sample toxins, the cytotoxicities of which have been evaluated and compared to values reported in the literature. Similar values have been reported using colorimetric assays; however, the simplicity of this electrochemical assay can, in principle, open the way to miniaturization onto lab-on-chip devices and its incorporation into tiered-testing approaches for cytotoxicity assessment.