Engineering Escherichia coli for enhanced sensitivity to the autoinducer-2 quorum sensing signal

Redox-mediated Biomolecular information transfer in single electrogenetic biological cells

Electronic communication in natural systems makes use, inter alia, of molecular transmission, where electron transfer occurs within networks of redox reactions, which play a vital role in many physiological systems. In view of the limited understanding of redox signaling, we developed an approach and an electrochemical-optical lab-on-a-chip to observe cellular responses in localized redox environments. The developed fluidic micro-system uses electrogenetic bacteria in which a cellular response is activated to electrically and chemically induced stimulations. Specifically, controlled environments for the cells are created by using microelectrodes to generate spatiotemporal redox gradients. The in-situ cellular responses at both single-cell and population levels are monitored by optical microscopy. The elicited electrogenetic fluorescence intensities after 210 min in response to electrochemical and chemical activation were 1.3 × 108±0.30 × 108 arbitrary units (A.U.) and 1.2 × 108±0.30 × 108 A.U. per cell population, respectively, and 1.05 ± 0.01 A.U. and 1.05 ± 0.01 A.U. per-cell, respectively. We demonstrated that redox molecules' mass transfer between the electrode and cells - and not the applied electrical field - activated the electrogenetic cells. Specifically, we found an oriented amplified electrogenetic response on the charged electrodes' downstream side, which was determined by the location of the stimulating electrodes and the flow profile. We then focused on the cellular responses and observed distinct subpopulations that were attributed to electrochemical rather than chemical stimulation, with the distance between the cells and the stimulating electrode being the main determinant. These observations provide a comprehensive understanding of the mechanisms by which diffusible redox mediators serve as electron shuttles, imposing context and activating electrogenetic responses.

Principles and challenges of whole cell microbial biosensors in the food industry

Whole cell microbial biosensors (WCMB) are mostly genetically modified microorganisms used to detect target molecules as indicators of biological and chemical contaminants as well as in the identification of compounds of interest in the food industry. The specificity and sensitivity of these biosensors are achieved through the design of genetic circuits that make use of genetic sequences such as promoters, terminators, genes encoding regulatory proteins or reporter proteins, among others. Despite the advances of WCMBs for their application, significant challenges are faced, such as cell stability, regulatory restrictions, and the need to optimize response times so that they can be a competitive detection tool in the market. This review explores the technological progress, potential and limitations of WCMBs in the food industry, starting by reviewing the operating principles of biosensors. The importance of selecting appropriate chassis cells and the integration of recognition elements and transducers to maximize their effectiveness in the detection of contaminants and compounds of interest in the food industry is highlighted.

A Simple Biosensor-Based Assay for Quantitative Autoinducer-2 Analysis

Bacteria produce and react to interspecies signaling molecules in order to control the expression of genes that are particularly beneficial when they are expressed by a bacterial community. In addition to intraspecies communication, the signaling molecule autoinducer-2 (AI-2) can also serve for interspecies communication between Gram-positive and Gram-negative bacteria and is therefore of particular interest. The analysis and quantification of AI-2 are essential for understanding population density-dependent changes in bacterial behavior and pathogenicity. However, currently available bioassays for AI-2 quantification are rather complex, have narrow detection ranges, and are very sensitive to trace components of, for example, growth media. To facilitate and improve the detection of AI-2, we have developed an Escherichia coli biosensor-based assay that is sensitive, cheap, fast, robust, and reliable in the quantification of biologically active AI-2. The bioassay is based on an lsr promoter-fluorescent reporter gene fusion cassette that we chromosomally integrated in a biosensor strain, but the cassette can also be used in a low-copy number plasmid for the application in other Gram-negative bacterial species. We show here that AI-2 quantification was possible in a concentration range from 400 nM to 100 μM and that a critical interpretation of the kinetics of the measurements can reveal sugar interference. With the help of our biosensor strain, coculture experiments were done to test the capability and kinetics of AI-2 secretion by various Gram-negative bacteria in real time. Finally, calibration curves were used to calculate the absolute AI-2 concentration in cell-free bacterial samples.

The Quorum Sensing Auto-Inducer 2 (AI-2) Stimulates Nitrogen Fixation and Favors Ethanol Production over Biomass Accumulation in Zymomonas mobilis

Autoinducer 2 (or AI-2) is one of the molecules used by bacteria to trigger the Quorum Sensing (QS) response, which activates expression of genes involved in a series of alternative mechanisms, when cells reach high population densities (including bioluminescence, motility, biofilm formation, stress resistance, and production of public goods, or pathogenicity factors, among others). Contrary to most autoinducers, AI-2 can induce QS responses in both Gram-negative and Gram-positive bacteria, and has been suggested to constitute a trans-specific system of bacterial communication, capable of affecting even bacteria that cannot produce this autoinducer. In this work, we demonstrate that the ethanologenic Gram-negative bacterium Zymomonas mobilis (a non-AI-2 producer) responds to exogenous AI-2 by modulating expression of genes involved in mechanisms typically associated with QS in other bacteria, such as motility, DNA repair, and nitrogen fixation. Interestingly, the metabolism of AI-2-induced Z. mobilis cells seems to favor ethanol production over biomass accumulation, probably as an adaptation to the high-energy demand of N₂ fixation. This opens the possibility of employing AI-2 during the industrial production of second-generation ethanol, as a way to boost N₂ fixation by these bacteria, which could reduce costs associated with the use of nitrogen-based fertilizers, without compromising ethanol production in industrial plants.

Homologous Quorum Sensing Regulatory Circuit: A Dual-Input Genetic Controller for Modulating Quorum Sensing-Mediated Protein Expression in E. coli

We developed a hybrid synthetic circuit that co-opts the genetic regulation of the native bacterial quorum sensing autoinducer-2 and imposes an extra external controller for maintaining tightly controlled gene expression. This dual-input genetic controller was mathematically modeled and, by design, can be operated in three modes: a constitutive mode that enables consistent and high levels of expression; a tightly repressed mode in which there is very little background expression; and an inducible mode in which concentrations of two signals (arabinose and autoinducer-2) determine the net amplification of the gene(s)-of-interest. We demonstrate the utility of the circuit for the controlled expression of human granulocyte macrophage colony stimulating factor in an engineered probiotic E. coli. This dual-input genetic controller is the first homologous AI-2 quorum sensing circuit that has the ability to be operated in three different modes. We believe it has the potential for wide-ranging biotechnological applications due its versatile features.