Role of reactive oxygen species and bacterial exopolymeric substances in manganese biomineralization

The aim of this PhD project was to determine what are the local physico-chemical factors that trigger the precipitation of manganese in biofilms using three mutants of Escherichia coli MG1655 and two of E. coli 1094 that produce different types of EPS. This work has shown that E. coli MG1655 is able to oxidize MnII by a yet unknown mechanism. We propose that this process is driven by superoxide anions produced by the bacteria and released in the biofilm porosity. We have shown that the strains of E. coli studied here produced superoxide anions and that globally biofilms were scavenging these reactive oxygen species. More specifically, we propose that in the biofilm porosity, superoxyde is produced by bacteria in their log phase, and scavenged in their late stationary phase. In a biofilm form, superoxides are also locally sequestrated in the porosity providing microenvironments with distinct superoxide concentrations. The oxidative activity is enhanced under light exposure, confirming the role of ROS in the mechanism. Spatial distribution of the formed oxidized Mn-rich nanoparticles is dependent on the exopolymer secreted and therefore suggests that the location of mineralization is governed by the nature of EPS. Such results are further confirmed using in situ experiments by transmission electron microscopy in a liquid cell highlighting the role of the density of charged functionalized sites on mineral localization, morphology and formation kinetics. These latter results bring new perspectives towards the identification of transient species that can direct mineralization on organic polymers and cell surface.