What do luminescent bacterial metal-sensors probe? Insights from confrontation between experiments and flux-based theory

Abstract

Whole-cell bioreporters are routinely operated as sentinels for monitoring the concentration of bioavailable and/or toxic metal ions (M) in aquatic media. Despite the importance of metal bioreporters in environmental risk assessment, their use is often limited to the establishment and exploitation of calibration curves relating bioreporters signal and target metal concentration. In this work, a physicochemical rationale is elaborated for the response of metal-sensitive whole-cell bioreporters beyond the restrictive representation of metal partitioning equilibrium at the microorganism-solution interface. The analysis is conducted under poorly metal complexing conditions for steady-state bioreporter functioning defined by a rate of photons production independent of time. The theoretical framework deciphers how this rate is determined by (i) metal biouptake dynamics with contributions from metal conductive diffusion to the cell surface and metal internalisation kinetics, (ii) formation kinetics and stability of intracellular complexes between M and transcriptional regulators, and (iii) the flux of emitted photons resulting from biochemical reactions initiated after activation of transcriptional regulators. The formalism enables quantitative evaluation of bioreporters performance depending on interfacial cell electrostatics, cell concentration and cell metal-adsorption features. The theory is supported by experimental data on cadmium detection by genetically modified luminescent Escherichia coli bioreporters exhibiting various lipopolysaccharidic surface structures.