Kinetics of metal detection by luminescence-based whole-cell biosensors: connecting biosensor response to metal bioavailability, speciation and cell metabolism

Luminescent whole-cell metal biosensors are genetically engineered cells used for the detection of metals in e.g. aqueous solutions. Herein, we detail the quantitative connections between time-response of luminescent bacterial metal sensors and the bioavailability of free and complexed metal species. To that end, we formulate the biophysicochemical dynamics of metal partitioning at a biosensor/solution interface and integrate the required metabolism contribution to cell response. The formalism explains the ways in which cell signal depends on: coupled Eigen kinetics of metal complexation and diffusion of metal species to/from the interface; kinetics of metal excretion, Michaelis-Menten bioaccumulation and ensuing metal depletion from bulk solution; and kinetics of bioluminescence production following intracellular metal sequestration by regulatory metalloproteins. In turn, an expression is derived for the time-dependent cell signal as a function of interrelated (bioavai)lability of metal species and (thermo)dynamic descriptors of extra/intracellular metal complexation. Quantitative criteria are elaborated to identify scenarios where equilibrium modeling of metal speciation is incorrect, bulk metal depletion is operative, metal biouptake kinetics is governed by metal diffusion, or labile metal complexes fully contribute to cell response. Remarkably, in agreement with experiments, the theory predicts time-shifts of bioluminescence peaks with increasing concentration of biosensor and/or metal ligand in solution. We show that these shifts originate from the crosstalk between activation kinetics of cell photoactivity and speciation-dependent kinetics of bulk metal depletion. Overall, the work paves the way for the elaboration of new strategies to exploit the bioluminescence response of metal lux-biosensors at a dynamic level and evaluate metal bioavailability properties in environmental or biological aqueous samples.

Effect of mercury in the influx and efflux of nutrients in the microalga Desmodesmus armatus

Anthropogenic activities such as mining and the metallurgical industry are the main sources of mercury contamination. Mercury is one of the most serious environmental problems in the world. This study aimed to investigate, using experimental kinetic data, the effect of different inorganic mercury (Hg²⁺) concentrations on the response of microalga Desmodesmus armatus stress. Cell growth, nutrients uptake and mercury ions from the extracellular medium, and oxygen production were determined. A Compartment Structured Model allowed elucidating the phenomena of transmembrane transport, including influx and efflux of nutrients, metal ions and bioadsorption of metal ions on the cell wall, which are difficult to determine experimentally. This model was able to explain two tolerance mechanisms against mercury, the first one was the adsorption of Hg²⁺ions onto the cell wall and the second was the efflux of mercury ions. The model predicted a competition between internalization and adsorption with a maximum tolerable concentration of 5.29 mg/L of HgCl₂. The kinetic data and the model showed that mercury causes physiological changes in the cell, which allow the microalga to adapt to these new conditions to counteract the toxic effects. For this reason, D. armatus can be considered as a Hg-tolerant microalga. This tolerance capacity is associated with the activation of the efflux as a detoxification mechanism that facilitates the maintenance of the osmotic balance for all the modeled chemical species. Furthermore, the accumulation of mercury in the cell membrane suggests the presence of thiol groups associated with its internalization, leading to the conclusion that metabolically active tolerance mechanisms are dominant over passive ones.

Electrostatics of soft (bio)interfaces: Corrections of mean-field Poisson-Boltzmann theory for ion size, dielectric decrement and ion-ion correlation

HYPOTHESIS

Electrostatics of soft (ion-permeable) (bio)particles (e.g. microorganisms, core/shell colloids) in aqueous electrolytes is commonly formulated by the mean-field Poisson-Boltzmann theory and integration of the charge contributions from electrolyte ions and soft material. However, the effects connected to the size of the electrolyte ions and that of the structural charges carried by the particle, to dielectric decrement and ion-ion correlations on soft interface electrostatics have been so far considered at the margin, despite the limits of the Gouy theory for condensed and/or multivalent electrolytes.

EXPERIMENTS

Accordingly, we modify herein the Poisson-Boltzmann theory for core/shell (bio)interfaces to include the aforementioned molecular effects considered separately or concomitantly. The formalism is applicable for poorly to highly charged particles in the thin electric double layer regime and to unsymmetrical multivalent electrolytes.

FINDINGS

Computational examples of practical interests are discussed with emphasis on how each considered molecular effect or combination thereof affects the interfacial potential distribution depending on size and valence of cations and anions, size of particle charges, length scale of ionic correlations and shell-to-Debye layer thickness ratio. The origins of here-evidenced pseudo-harmonic potential profile and ion size-dependent screening of core/shell particle charges are detailed. In addition, the existence and magnitude of the Donnan potential when reached in the shell layer are shown to depend on the excluded volumes of the electrolyte ions.

Exploiting Catabolite Repression and Stringent Response to Control Delay and Multimodality of Bioluminescence Signal by Metal Whole-Cell Biosensors: Interplay between Metal Bioavailability and Nutritional Medium Conditions

The time-dependent response of metal-detecting whole-cell luminescent bacterial sensors is impacted by metal speciation/bioavailability in solution. The comprehensive understanding of such connections requires the consideration of the bacterial energy metabolism at stake and the effects of supplied food on cells’ capability to convert bioaccumulated metals into light. Accordingly, we investigated the time response (48 h assay) of PzntA-luxCDABE Escherichia coli Cd biosensors in media differing with respect to sources of amino acids (tryptone or Lysogeny Broth) and carbon (glucose, xylose and mixtures thereof). We show that the resulting coupling between the stringent cell response and glucose/xylose-mediated catabolite repressions lead to well-defined multimodalities and shapes of the bioluminescence signal over time. Based on a recent theory for the time–response of metal-sensing luminescent bacteria, successful theoretical reconstructions of the bioluminescence signals are reported under all Cd concentrations (0–20 nM) and nutritive conditions examined. This analysis leads to the evaluation of time-dependent cell photoactivity and qualitative information on metal speciation/bioavailability in solution. Biosensor performance and the position, shape, number, and magnitude of detected peaks are discussed in relation to the metabolic pathways operative during the successive light emission modes identified here over time. Altogether, the results clarify the contributions of metal/nutrient bio-availabilities and food quality to cell response typology.

Brush Layer Charge Characteristics of a Biomimetic Polyelectrolyte-Modified Nanoparticle Surface

Nanoparticle surface charge regulation technology plays an important role in ion rectification, drug delivery, and cell biology. The biomimetic polyelectrolyte can be combined with nanoparticles by nanomodification technology to form a layer of coating, which is called the brush layer of nanoparticles. In this study, based on the Poisson-Nernst-Planck (PNP) equation system, a theoretical model considering a bionic electrolyte brush layer with charge density regulated by a chemical reaction is constructed. The charge properties of brushed nanoparticles are studied by changing the sizes of nanoparticles, the pH value of the solution, background salt solution concentration, and brush layer thickness. The result shows that the charge density of brushed nanoparticles increases with the increase of particle size. The isoelectric point (IEP) of the equilibrium reaction against the brush layer is pH = 5.5. When the pH < 5.5, the charge density of the particle brush layers decreases with the increase of pH, and when the pH > 5.5, the charge density of the particle brush layer increases with the increase of pH. By comparing the charge density of different brush thicknesses, it is found that the larger the brush thickness, the smaller the charge density of the brush layer. This research provides theoretical support for the change of the through pore velocity when macromolecular organic compounds pass through nanopores.

Charge Properties and Electric Field Energy Density of Functional Group-Modified Nanoparticle Interacting with a Flat Substrate

Functionalized nanofluidics devices have recently emerged as a powerful platform for applications of energy conversion. Inspired by biological cells, we theoretically studied the effect of the interaction between the nanoparticle and the plate which formed the brush layer modified by functional zwitterionic polyelectrolyte (PE) on the bulk charge density of the nanoparticle brush layer, and the charge/discharge effect when the distance between the particle and the plate was changed. In this paper, The Poisson-Nernst-Planck equation system is used to build the theoretical model to study the interaction between the nanoparticle and the plate modified by the PE brush layer, considering brush layer charge regulation in the presence of multiple ionic species. The results show that the bulk charge density of the brush layer decreases with the decrease of the distance between the nanoparticle and the flat substrate when the interaction occurs between the nanoparticle and the plate. When the distance between the particle and the plate is about 2 nm, the charge density of the brush layer at the bottom of the particle is about 69% of that at the top, and the electric field energy density reaches the maximum value when the concentration of the background salt solution is 10 mm.

Addressing the electrostatic component of protons binding to aquatic nanoparticles beyond the Non-Ideal Competitive Adsorption (NICA)-Donnan level: Theory and application to analysis of proton titration data for humic matter

HYPOTHESIS

Charge descriptors of aquatic nanoparticles (NPs) are evaluated from proton titration curves measured at different salt concentrations and routinely analysed by the Non-Ideal Competitive Adsorption-Donnan (NICAD) model. This model, however, suffers from approximations regarding particle electrostatics, which may bias particle charge estimation. Implementation of Poisson-Boltzmann (PB) theory within consistent treatment of NPs protolytic data is expected to address NICAD shortcomings.

EXPERIMENTS

An alternative to NICAD is elaborated on the basis of nonlinearized PB equation for soft particle electrostatics to properly unravel the electrostatic and chemical components of proton binding to NPs. A numerical package is developed for automated analysis of proton titration curves and proton affinity spectra at different salt concentrations. The performance of the method is illustrated for humic matter nanoparticles with different charge and size, and compared to that of NICAD.

FINDINGS

Unlike NICAD, PB-based treatment successfully reproduces particle charge dependence on pH for practical salt concentrations from the thin to thick electric double layer limit. Donnan representation in NICAD leads to moderate to dramatic misestimations of proton affinity and binding heterogeneity depending on particle size to Debye layer thickness ratio. Interpretation of NPs protolytic properties with PB theory further avoids adjustment of the 'particle Donnan volume' empirically introduced in NICAD.

Electrophoresis of composite soft particles with differentiated core and shell permeabilities to ions and fluid flow

Within the framework of analytical theories for soft surface electrophoresis, soft particles are classically defined by a hard impermeable core of given surface charge density surrounded by a polyelectrolyte shell layer permeable to both electroosmotic flow and ions from background electrolyte. This definition excludes practical core-shell particles, e.g. dendrimers, viruses or multi-layered polymeric particles, defined by a polyelectrolytic core where structural charges are distributed and where counter-ions concentration and electroosmotic flow velocity can be significant. Whereas a number of important approximate expressions has been derived for the electrophoretic mobility of hard and soft particles, none of them is applicable to such generic composite core-shell particles with differentiated ions- and fluid flow-permeabilities of their core and shell components. In this work, we elaborate an original closed-form electrophoretic mobility expression for this generic composite particle type within the Debye-Hückel electrostatic framework and thin double layer approximation. The expression explicitly involves the screening Debye layer thickness and the Brinkman core and shell hydrodynamic length scales, which favors so-far missing analysis of the respective core and shell contributions to overall particle mobility. Limits of this expression successfully reproduce results from Ohshima's electrophoresis theory solely applicable to soft particles with or without hard core.

Rigorous Physicochemical Framework for Metal Ion Binding by Aqueous Nanoparticulate Humic Substances: Implications for Speciation Modeling by the NICA-Donnan and WHAM Codes

Latest knowledge on the reactivity of charged nanoparticulate complexants toward aqueous metal ions is discussed in mechanistic detail. We present a rigorous generic description of electrostatic and chemical contributions to metal ion binding by nanoparticulate complexants, and their dependence on particle size, particle type (i.e., reactive sites distributed within the particle body or confined to the surface), ionic strength of the aqueous medium, and the nature of the metal ion. For the example case of soft environmental particles such as fulvic and humic acids, practical strategies are delineated for determining intraparticulate metal ion speciation, and for evaluating intrinsic chemical binding affinities and heterogeneity. The results are compared with those obtained by popular codes for equilibrium speciation modeling (namely NICA-Donnan and WHAM). Physicochemical analysis of the discrepancies generated by these codes reveals the a priori hypotheses adopted therein and the inappropriateness of some of their key parameters. The significance of the characteristic time scales governing the formation and dissociation rates of metal-nanoparticle complexes in defining the relaxation properties and the complete equilibration of the metal-nanoparticulate complex dispersion is described. The dynamic features of nanoparticulate complexes are also discussed in the context of predictions of the labilities and bioavailabilities of the metal species.

Decoding the Time-Dependent Response of Bioluminescent Metal-Detecting Whole-Cell Bacterial Sensors

The signal produced by aqueous dispersions of bioluminescent, metal-responsive whole-cell bacterial sensors is indicative of the concentration of bioavailable metal ions in solution. The conventional calibration-based strategy followed for measuring this concentration is however inadequate to provide any quantitative prediction of the cell response over time as a function of, e.g., their growth features, their defining metal accumulation properties, or the physicochemical medium composition. Such an evaluation is still critically needed for assessing on a mechanistic level the performance of biosensors in terms of metal bioavailability and toxicity monitoring. Herein we report a comprehensive formalism unraveling how the dependence of bioluminescence on time is governed by the dynamics of metal biouptake, by the activation kinetics of lux-based reporter gene, and by the ensuing rate of luciferase production, the kinetics of light emission, and quenching. It is shown that the bioluminescence signal corresponds to the convolution product between two time-dependent functions, one detailing the dynamic interplay of the above micro- and nanoscale processes, and the other pertaining to the change in concentration of photoactive cell sensors over time. Numerical computations illustrate how the shape and magnitude of the bioluminescence peak(s) are intimately connected to the dependence of the photoactive cell concentration on time and to the magnitudes of Deborah numbers that compare the relevant time scales of the biointerfacial and intracellular events controlling light emission. Explicit analytical expressions are further derived for practical situations where bioluminescence is proportional to the concentration of metal ions in solution. The theory is further quantitatively supported by experiments performed on luminescent cadmium-responsive lux-based Escherichia coli biosensors.

Effect of ion partitioning on electrostatics of soft particles with volumetrically charged inner core coated with pH-regulated polyelectrolyte layer

The effect of ion partitioning on the electrostatics of a soft particle with a volumetrically charged core and a pH-dependent polyelectrolyte layer (PEL) is numerically investigated. It is observed that, whenever the ion partitioning is noticeable, the soft layer can be fully charged in a broader range of pH. Besides, a higher number density of the PEL functional groups and a lower charge density of the core result in a sharper dependence of the electric potential on the electrolyte pH. Briefly, we conclude that, since the PEL charge is dependent upon the concentration of the hydroxide/hydrogen ions, for the pH-regulated soft particles, the ion partitioning effect, as a phenomenon influencing the ionic distribution, can be a determinant factor. So taking the effect of the ion partitioning into consideration is strongly recommended for a more realistic description of the electrostatics of the pH-regulated soft particles.

Impact of intracellular metallothionein on metal biouptake and partitioning dynamics at bacterial interfaces

This study reports the quantitative evaluation of the metal biopartitioning dynamics following biouptake at bacterial interfaces with explicit account of the effects stemming from intracellular metal binding by metallothionein proteins.

Kinetic and thermodynamic determinants of trace metal partitioning at biointerphases: the role of intracellular speciation dynamics

A theory is elaborated for rationalizing the impacts of intracellular metal speciation dynamics on metal uptake in suspension of charged microorganisms beyond the classical thermodynamic representation.

Coupled metal partitioning dynamics and toxicodynamics at biointerfaces: a theory beyond the biotic ligand model framework

A theory is developed for coupled toxicodynamics and interfacial metal partitioning dynamics, with integration of intertwined metal adsorption–internalisation–excretion-transport at the biointerface, cell growth and metal depletion from solution.

Dynamics of Metal Partitioning at the Cell-Solution Interface: Implications for Toxicity Assessment under Growth-Inhibiting Conditions

Metal toxicity toward microorganisms is usually evaluated by determining growth inhibition. To achieve a mechanistic interpretation of such toxic effects, the intricate coupling between cell growth kinetics and metal partitioning dynamics at the cell-solution interface over time must be considered on a quantitative level. A formalism is elaborated to evaluate cell-surface-bound, internalized, and extracellular metal fractions in the limit where metal uptake kinetics is controlled by internalization under noncomplexing medium conditions. Cell growth kinetics is tackled using the continuous logistic equation modified to include growth inhibition by metal accumulation to intracellular or cell surface sites. The theory further includes metal-proton competition for adsorption at cell-surface binding sites, as well as possible variation of cell size during exposure to metal ions. The formalism elucidates the dramatic impacts of initial cell concentration on metal bioavailability and toxicity over time, in agreement with reported algae bioassays. It further highlights that appropriate definition of toxicity endpoints requires careful inspection of the ratio between exposure time scale and time scale of metal depletion from bulk solution. The latter depends on metal internalization-excretion rate constants, microorganism growth, and the extent of metal adsorption on nonspecific, transporter, and growth inhibitory sites. As an application of the theory, Cd toxicity in the algae Pseudokirchneriella subcapitata is interpreted from constrained modeling of cell growth kinetics and of interfacial Cd-partitioning dynamics measured under various exposure conditions.

Evaluation of metal biouptake from the analysis of bulk metal depletion kinetics at various cell concentrations: theory and application

Bioavailability of trace metals is a key parameter for assessment of toxicity on living organisms. Proper evaluation of metal bioavailability requires monitoring the various interfacial processes that control metal partitioning dynamics at the biointerface, which includes metal transport from solution to cell membrane, adsorption at the biosurface, internalization, and possible excretion. In this work, a methodology is proposed to quantitatively describe the dynamics of Cd(II) uptake by Pseudomonas putida. The analysis is based on the kinetic measurement of Cd(II) depletion from bulk solution at various initial cell concentrations using electroanalytical probes. On the basis of a recent formalism on the dynamics of metal uptake by complex biointerphases, the cell concentration-dependent depletion time scales and plateau values reached by metal concentrations at long exposure times (>3 h) are successfully rationalized in terms of limiting metal uptake flux, rate of excretion, and metal affinity to internalization sites. The analysis shows the limits of approximate depletion models valid in the extremes of high and weak metal affinities. The contribution of conductive diffusion transfer of metals from the solution to the cell membrane in governing the rate of Cd(II) uptake is further discussed on the basis of estimated resistances for metal membrane transfer and extracellular mass transport.

Uranium accumulation and toxicity in the green alga Chlamydomonas reinhardtii is modulated by pH

The effects of pH on metal uptake and toxicity in aquatic organisms are currently poorly understood and remain an evolving topic in studies about the biotic ligand model (BLM). In the present study, the authors investigated how pH may influence long-term (4 d) uranium (U) accumulation and chronic toxicity in batch cultures of the freshwater green alga Chlamydomonas reinhardtii. The toxicity expressed as a function of the free uranyl ion was much greater at pH 7 (effective concentration, 50% [EC50] = 1.8 × 10(-9)  M UO2 (2+) ) than at pH 5 (EC50 = 1.2 × 10(-7)  M UO2 (2+) ). The net accumulation rate of U in algal cells was much higher at pH 7 than at pH 5 for the same free [UO2 (2+) ], but the cells exposed at pH 5 were also more sensitive to intracellular U than the cells at pH 7 with EC50s of 4.0 × 10(-15) and 7.1 × 10(-13)  mol of internalized U cell(-1) , respectively. The higher cellular sensitivity to U at pH 5 than at pH 7 could be explained partly by the increase in cytosolic U binding to algal soluble proteins or enzymes at pH 5 as observed by subcellular fractionation. To predict U accumulation and toxicity in algae accurately, the important modulating effects of pH on U accumulation and U cellular sensitivity should be considered in the BLM.

Dynamics of metal uptake by charged soft biointerphases: impacts of depletion, internalisation, adsorption and excretion

A formalism is elaborated for deriving the dynamic partitioning of metal ions across an active, charged soft biointerphase with full account of metal depletion, transport, internalisation, adsorption and excretion processes.