Widespread bacterial responses and their mechanism of bacterial metallogenic detoxification under high concentrations of heavy metals

Microbial mineralization is increasingly used in bioremediation of heavy metal pollution, but better mechanistic understanding of the processes involved and how they are regulated are required to improve the practical application of microorganisms in bioremediation. We used a combination of morphological (TEM) and analytical (XRD, XPS, FTIR) methods, together with novel proteomic analyses, to investigate the detoxification mechanisms, used by a range of bacteria, including the strains Bacillus velezensis LB002, Escherichia coli DH5α, B. subtilis 168, Pseudomonas putida KT2440, and B. licheniformis MT-1, exposed to elevated concentrations of Cd2+ and combinations of Cd2+, Pb2+, Cu2+, and Zn2+, in the presence and absence of added CaCl2. Common features of detoxification included biomineralization, including the production of biological vaterite, up-regulation of proteins involved in flagellar movement and chemotaxis, biofilm synthesis, transmembrane transport of small molecules and organic matter decomposition. The putative roles of differentially expressed proteins in detoxification are discussed in relation to chemical and morphological data and together provide important tools to improve screening, selection, and practical application of bacterial isolates in bioremediation of polluted environments.