Electrical Control of Escherichia coli Growth Measured with Simultaneous Modulation and Imaging

Label-free detection of E. coli by alternating current electrokinetic capacitive sensors

BACKGROUND

Escherichia coli (E. coli) is one of the most common pathogens in water and foods, and it poses a significant threat to global public health, particularly in low- and middle-income countries. However, most of the state-of-art detection method for E. coli bacteria sensing needs fluorescence staining or pre-immobilizations or specific biomaker as labels before tests, making the detection process a time costing and labor intensitive work. Thus, it is desirable to develop a label-free and no-immobilization detection method for the sensitive detection of E. coli or other pathogens.

RESULTS

In this work, we developed a sensitive method for E. coli detection without any labels or pre-immobilizations. Here the source AC signal provides ACEK effects to trap bacteria onto the electrode surface. As the source AC signal is cut off, the enriched bacteria would release from sensor electrodes. Then, a significant interfacial capacitance change along with particle releasing is observed by applying a measurement AC signal. The DEP capture of E. coli bacteria at different AC frequencies and voltages are experimentally investigated to optimize the performance of capacitive sensors. To note, the total area of our sensitive unit can be as large as 2 mm × 2 mm with the geometry design of isomotive DEP. Capacitive sensing tests of river water collected from local river and tap water with several control groups are performed to show its reliability as a biosensing approach for the sensitive detection of E. coli.

SIGNIFICANCE

The most significant advantage of this method is that we do not need to pre-immobilize any probes (antigens, or antibodies, etc.) or take incubation onto the sensor electrode before taking capacitance measurement. Moreover, the straight forward operation is pre-immobilization free, and therefore it meets the requirements of on-site bacteria detection for pathogen detection in water quality monitoring or food safety.

From the Microbiome to the Electrome: Implications for the Microbiota-Gut-Brain Axis

The gut microbiome plays a fundamental role in metabolism, as well as the immune and nervous systems. Microbial imbalance (dysbiosis) can contribute to subsequent physical and mental pathologies. As such, interest has been growing in the microbiota-gut-brain brain axis and the bioelectrical communication that could exist between bacterial and nervous cells. The aim of this study was to investigate the bioelectrical profile (electrome) of two bacterial species characteristic of the gut microbiome: a Proteobacteria Gram-negative bacillus Escherichia coli (E. coli), and a Firmicutes Gram-positive coccus Enterococcus faecalis (E. faecalis). We analyzed both bacterial strains to (i) validate the fluorescent probe bis-(1,3-dibutylbarbituric acid) trimethine oxonol, DiBAC4(3), as a reliable reporter of the changes in membrane potential (Vmem) for both bacteria; (ii) assess the evolution of the bioelectric profile throughout the growth of both strains; (iii) investigate the effects of two neural-type stimuli on Vmem changes: the excitatory neurotransmitter glutamate (Glu) and the inhibitory neurotransmitter γ-aminobutyric acid (GABA); (iv) examine the impact of the bioelectrical changes induced by neurotransmitters on bacterial growth, viability, and cultivability using absorbance, live/dead fluorescent probes, and viable counts, respectively. Our findings reveal distinct bioelectrical profiles characteristic of each bacterial species and growth phase. Importantly, neural-type stimuli induce Vmem changes without affecting bacterial growth, viability, or cultivability, suggesting a specific bioelectrical response in bacterial cells to neurotransmitter cues. These results contribute to understanding the bacterial response to external stimuli, with potential implications for modulating bacterial bioelectricity as a novel therapeutic target.

Effect of Thioflavin T on the Elongation Rate of Bacteria

Background

The growing field of bacterial electrophysiology examines the relationship between bacterial membrane potential and cell division, growth, sporulation, and biofilm formation. These experiments require Nernstian fluorescent dyes to monitor membrane potential. Our research uses single cell imaging to determine if a common fluorescent dye, Thioflavin T (ThT), affects the growth of bacteria.

Materials and Methods

We use a combination of standard growth curve measurements and single cell imaging, both brightfield and fluorescence microscopy, to monitor the growth of Bacillus subtilis and Escherichia coli as a function of ThT concentration. Increased membrane potential (hyperpolarization) leads to increased intracellular accumulation of ThT: High fluorescence intensity is an indicator of hyperpolarization. Blue light is used to hyperpolarize a subpopulation of cells to monitor cellular elongation in response to increased cellular internalization of ThT.

Results

Single cell imaging shows that the elongation rates of B. subtilis and E. coli are decreased when these cells are incubated with ThT. At micromolar concentrations of ThT, this effect may be masked in standard growth curves, but is visible with single cell measurements on agarose pads.

Conclusions

The increased cellular accumulation of ThT is a standard measure of hyperpolarization in bacterial electrophysiology. Growth curves, a bulk measurement, are typically used to determine suitable concentrations of ThT for use in experiments. Single cell measurements show that cells incubated with ThT have decreased elongation rates. This creates a potential experimental artifact that could lead to misinterpretation of data. Hyperpolarized cells internalize more ThT. This increased intracellular concentration of ThT, rather than the change in membrane potential, could lead to decreased growth. These experiments point toward the importance of single cell measurements to detect subtle changes in cell growth. We hope this research will be useful for other researchers in their choice of dye for the detection of membrane potential.

Current Progress of Interfacing Organic Semiconducting Materials with Bacteria

Microbial bioelectronics require interfacing microorganisms with electrodes. The resulting abiotic/biotic platforms provide the basis of a range of technologies, including energy conversion and diagnostic assays. Organic semiconductors (OSCs) provide a unique strategy to modulate the interfaces between microbial systems and external electrodes, thereby improving the performance of these incipient technologies. In this review, we explore recent progress in the field on how OSCs, and related materials capable of charge transport, are being used within the context of microbial systems, and more specifically bacteria. We begin by examining the electrochemical communication modes in bacteria and the biological basis for charge transport. Different types of synthetic organic materials that have been designed and synthesized for interfacing and interrogating bacteria are discussed next, followed by the most commonly used characterization techniques for evaluating transport in microbial, synthetic, and hybrid systems. A range of applications is subsequently examined, including biological sensors and energy conversion systems. The review concludes by summarizing what has been accomplished so far and suggests future design approaches for OSC bioelectronics materials and technologies that hybridize characteristic properties of microbial and OSC systems.