Cd²⁺ tolerance and removal mechanisms of Serratia marcescens KMR-3

Heavy metal contamination is a global issue, with cadmium (Cd²⁺) and its treatment becoming major environmental challenge that could be solved by microbial restoration, an eco-friendly technique. Serratia marcescens KMR-3 exhibits high tolerance and removal rate of Cd²⁺ (≤500 mg/L). Here, we aimed to explore mechanisms underlying tolerance to and removal of Cd²⁺ by KMR-3. Scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry were conducted to analyze characteristics of the KMR-3 biofilm and Cd²⁺ combined forms. The results revealed varying degrees of cell adhesion, membrane thickening, and shrinkage on the surface of the bacteria. The binding elements, electronic binding energy, and functional groups on the surface of the bacteria exhibited changes. Furthermore, the biofilm amount following treatment with Cd²⁺ was 1.5-3 times higher than that in the controls, treatment with Cd²⁺ substantially enhanced biofilm generation and increased Cd²⁺ adsorption. Cd²⁺ adsorption by its own secondary metabolite prodigiosin produced by KMR-3 was enhanced by 19.5 % compared with that observed without prodigiosin. Through transcriptome sequencing and RT-qPCR, we observed that Znu protein-chelating system regulated gene expression (znuA, znuB, and znuC), and the efflux mechanism of the P-type ATPase regulated the expression of genes (zntA, zntB, and zntR), which were significantly enhanced. Through the combined action of various strategies, KMR-3 demonstrated a high tolerance and removal ability of Cd²⁺, providing a theoretical basis to treat Cd²⁺ pollution.